Also, can someone tell me where to start from for Indian philosophy and culture like some map which covers all Philosophies?
Buddhism declined from India in the past, due to assimilation.
Firstly, Buddhism had flourished in ancient India, Pakistan and Afghanistan, but eventually, the Buddhist locals became subject to "hard" assimilation into Islam by foreign invaders. See the Wikipedia articles Buddhism in Afghanistan, Buddhism in Pakistan and also this article section. This is not unlike the disappearance of Zoroastrianism from its homeland of Persia.
Secondly, Buddhism suffered from "soft" assimilation into Hinduism.
Before Buddhism (around 500 BCE), the historic Vedic religion was mainly focused on sacrifices and rituals. The Upanishadic movement rose in India from 500 - 200 BCE. After this, the Vedanta movement rose as part of Classical Hinduism from 200 BCE - 1100 CE. The Hindu text Brahma Sutras that refutes Buddhist and other Nastika teachings, was written between 200 BCE and 400 CE. These are scholarly dates from Wikipedia.
Adi Shankara, the famous reformer who revived Hinduism and consolidated Advaita Vedanta, lived around 800 CE. Advaita is the philosophical school of Vedanta that is closest to Buddhism. Even in those days, Hindu philosophical opponents of Shankara accused him of being a crypto-Buddhist. Through Advaita Vedanta, Hinduism assimilated Buddhism. Some Hindus even claim that Mahayana's emptiness (shunyata) is exactly the same as Advaita's Transcendental Ultimate Reality.
On the other hand, the Dvaita Vedanta school, which has completely incompatible philosophy to Buddhism, assimilated Buddhism by making Buddha an incarnation of Vishnu. The Hindu text Bhagavata Purana written between 500 CE and 1000 CE (scholarly dates according to Wikipedia) states that Vishnu incarnated as Buddha to delude the immoral atheists into living ethical lives. According to ISKCON, the Buddha tricked atheists into taking refuge in the Buddha and serving the Buddha, who is actually Vishnu in disguise. More info on this is available on this page.
The final reason for Buddhism's "soft" assimilation into Hinduism is the development of concepts in Mahayana Buddhism that sound very similar to concepts in Hinduism, like the Eternal Buddha, Adi Buddha (Primordial Buddha), Buddha Nature (tathagatagarbha), storehouse consciousness (alayavijnana) and divine beings like Avalokiteshvara etc. These concepts may have arisen out of assimilation of Hindu concepts into Mahayana Buddhism.
Buddhism survived in the world through three routes - Tibetan Buddhism (and the preservation of the Tibetan Buddhist canon), East Asian Buddhism (through transmission of Mahayana texts to China and later to Japan) and Theravada Buddhism's revival in Sri Lanka through Buddhism's own version of Adi Shankara, Buddhaghosa.
Buddhism has experienced a limited revival in India due to three reasons. The first is the conversion of low caste Dalits to Buddhism by Dr. Ambedkar. The second is the popularization of Vipassana meditation by S. N. Goenka (who learnt it from a Buddhist monk in Burma). The third is the relocation of the Dalai Lama to Dharamsala, India.
Jainism has always been a small minority dharma and never achieved the prominence achieved by Buddhism. So one cannot say that Jainism has declined.
The Buddhist case is different. Hindus did not destroy Buddhism. Buddhism collapsed due to internal weakness and timely Hindu reforms. Buddhism lost out in a free trade of ideas a long time ago. I do not accept the claim that the Buddhism was the majority religion in India. It is true that Buddhism was a major challenge to Hinduism in the period 500 BCE to 500 CE.
There are many reasons for the collapse of the Buddhist challenge or the Hindu refusal to accept Buddhism. This answer may seem strange since most people are now influenced by western scholarly claims that Buddhism tried to reform the Hindu system. Actually the truth is that there were 2 reform movements in Indian society at about the same time (around 500 bce) directed against the ancient Vedic ritualists (those who follow the Vedic Karma kanda). The Buddhist reform movement is well known. It downgrades the Vedas and theism, links Varna to conduct and emphasizes self-effort. What is not well understood, mainly because of western scholars and westernized Hindu scholars, is that the Itihasa literature (specially Mahabharata and the Gita) also tried to reform the ancient Vedic ritualism. Just read these shlokas:
O Arjuna! There are people who delight in the eulogistic statements of the Vedas and argue that the purport of the Vedas consists in these and nothing else. They are full of worldly desires; paradise is their highest goal; and they are totally blind in a spiritual sense. They expatiate upon those florid Vedic texts which describe the means for the attainment of pleasure and power, which provide attractive embodiments as the fruits of actions, and which are full of descriptions of rites and rituals (through which these fulfilments are obtained). In the minds of these votaries of pleasure and power, addicted to enjoyments of the above description, steadfast wisdom (capable of revealing the truth) is never generated.
This is one of the fiercest attacks on the popular Hindu Dharma of the time. Buddhist scripture is actually more restrained. The next shloka that one must read is the following:
O son of Pritha! Taking refuge in Me, women, Vaisyas, Sudras, and likewise even men of socially handicapped birth, attain to the highest spiritual goal.
In my opinion the attitude shown in this shloka proved disastrous to the fortune of Buddhism in India. To understand the reason for my assertion I must first talk about my Sri Lankan Buddhist friend Ananda. One day Ananda showed me an image of a Goddess in his house. He has bought it in a Ratha Yatra fare of ISCKON held in the Santa Monica Beach. I was taken aback and asked him about the reason for his interest in the Goddess. He told me that he prays to Hindu deities since they can help during times of distress. I wanted to know why he doesn't pray to Buddha. He told me, 'What would be the point? How can a mere man like the Buddha help another man? Only gods and goddesses can help during times of distress.' With this in mind just think of Buddha's austere message to his disciple Ananda, 'Atmo Deepo Bhava' (Be like a lamp unto yourself). Compare that message of self reliance with the message in Gita 9.32 of God Vishnu, in charge of the universe, taking a human form and personally saving all who love Him. Buddhism has never fared well with theistic competitor. It was blown away by Hindu theism. Of course Gita 9.32 is important on its own. It is one of the biggest steps to reform Hinduism. Hinduism was extended to embrace more than 50 % of the population (women of all castes) and Hindus of lower castes, people who were looked down upon by the Vedic ritualists. Actually Buddha was reluctant to allow women into monkhood. He only agreed after persuaded by Ananda.
I am posting a passage from a famous 14th century text which shows the equal rights enjoyed by women in Hinduism.
Renunciation is mentioned in the Taittiriya and other Upanishads: 'Some have attained immortality, not by acts, nor by offspring, nor by wealth, but by renunciation alone' (Kaivalya Upnaishad., 3). Females also are entitled to this kind of renunciation. The Caturdharike of the Mokshadharma, by using the word bhikshuki (female mendicant) with reference to the lady in question, when dealing with the controversy between Sulabha and janaka, indicated that females may renounce before marriage, or after the death of their husband and may go about as religious mendicants, may learn and hear the sastras dealing with moksha (namely the Upanishad and cognate literature), may meditate upon the atman in seclusion and assume the emblems of tri-danda, etc. In the wake of the argument in the Devatadhikarana (the section about devata) in the fourth chapter of the third book of the Sarirakabhasya, wherein a discussion has been started relating to the rights of a widower (to such renunciation), the name of the lady Vacaknavi has also been mentioned. These references go to strengthen the right claimed by Maitreyi, the wife of Yajnavalkya, of whom she inquires in the words: 'Of what use would such (wealth) be to me, if the possession of such would not render me immortal? Tell me whatever your worship knows,as to what I should do, in order to be free from mortality' (Brihadaranyaka Upanishad 4.5.4).
Jivanmuktiviveka of Vidyaranya translated by S. Subrahmanya Sastri and T.R. Srinivasa Ayangar, Chapter 1, Vividisa-samnyasa: The Renunciation of the Seeker
Gita of course does not stop there. Gita in 18.41-47 links the idea of Varna to guna and karma. The last name based caste system is a jati (community) system and is not Varna based system discussed in Hindu scripture. I am posting the relevant Gita verses on varna:
O great hero! The duties of Brahmanas, Ksatriyas, Vaisyas and also Sudras have been divided according to the quality born of their own nature.
Serenity, control of the sense, austerity, purity, straight-forwardness, knowledge, insight, and faith in the Supreme Being - these are a Brahman's duties born of his own nature.
Prowess, splendor of personality, unfailing courage, resourcefulness, dauntless in battle, generosity, leadership - these are a Ksatriya's duties born of his specific nature.
Agriculture, cattle-rearing and trade form the duty of the Vaisya springing from his own nature, while the natural duty of a Sudra consists in subordinate service under others.
By being devoted to one's own natural duty, man attains to spiritual competency. Now hear how devotion to one's own natural duty generates spiritual competency.
From whom all beings have emanated and by whom all this universe is pervaded - by worshiping Him through the dedicated performance of one's duty, man attains to spiritual competency.
One's own duty, even if without excellence (i.e. inferior in the scale of worldly values)is more meritorious spiritually than the apparently well-performed duty of another. For no sin is incurred by one doing works ordained according to one's nature.
These verses do not support the last name based hereditary Hindu caste system which led to division in Hindu society. Gita clearly states that Varna is based on karma and guna. It is unfortunate that this message had to wait till 1900 ce when Vivekananda explained it.
Buddhism has also been criticized for weakening Indian society. Swami Vivekananda was specially critical of the monastery building spree which in his view weakened the nation.
'I do not believe many of his doctrines; of course, I do not. I believe that the Vedantism of the old Hindus is much more thoughtful, is a grander philosophy of life. ...............By and by, there arose huge temples and all the paraphernalia. The use of images was unknown before then. I say they were the first to use images. There are images of Buddha and all the saints, sitting about and praying. All this paraphernalia went on multiplying with this organisation. Then these monasteries became rich. The real cause of the downfall is here. Monasticism is all very good for a few; but when you preach it in such a fashion that every man and woman who has a mind immediately gives up social life, when you find over the whole of India monasteries, some containing a hundred thousand monks, sometimes twenty thousand monks in one building - huge gigantic buildings, these monasteries, scattered all over India and, of course, centres of learning, and all that - who were left to procreate progeny, to continue the race? Only the weaklings. All the strong and vigorous minds went out. And then came national decay by the sheer loss of vigor'
Complete Works of Swami Vivekananda, volume 3, Buddhistic India
There are also at least 3 major issues and 1 minor issue with Buddhism from the Hindu point of view.
Buddhist Karma theory is unstable without an Isvara (God) to orchestrate the rewards and punishment. After all the Karmic arrow is insentient and can not by itself decide on the morality of an action. The Karmic arrow can not be compared with the law of gravity since gravity does not have to decide on moral issues.
Buddhism also gave rise to atheistic tendencies. Hindus who turned towards atheism could follow Lokayats and did not need Buddhist atheism.
The other major problem is with the absence of Atman in Buddhist thought. The lack of an Atman would create the problem of instability regarding the personality of an individual. Shankara attacks the philosophical position of the various Buddhist schools and shows the validity of the Atman principle. I will quote Sankara's Brahma Sutra Bhasya II.ii.31 where he attacks the Buddhist Idealists (Vijnanavada), Buddhist Realists (Sarvastitvavadins) and Buddhist Nihilists (Sarvasunyavadins or Madhyamikas) to give a flavor of his argument:
As for the ego-consciousness that is assumed to be the abode of disposition (or tendency), that too has no stable form, since you postulate its momentariness like sense-perception. Hence it cannot be the abode of tendencies. For unless there be some principle running through everything and abiding through all the three periods of time or some unchanging witness of all, there can be no human dealing involving remembrance, recognition, etc, which are contingent on past impressions that are stored up in conformity with environment, time and causation. If the ego-consciousness be (assumed to be) unchanging by nature, your doctrine (of momentariness) will be set at naught. Moreover since the theory of momentariness is upheld equally in Vijnanavada, all the defects arising from momentariness that were levelled (by us) against the theory of these (Buddhists) who believe in the existence of (momentary) external things, viz those shown under the aphorisms starting from, "And because the earlier is negated when the later emerges" (II.ii.20) are to be remembered in this context as well. Thus are refuted both these Buddhist points of view - of both those who believe in external things and those who believe in (subjective) consciousness). As for the view of the absolute nihilist, no attempt is made for its refutation since it is opposed to all means of valid knowledge. For human behaviour, conforming as it does to all right means of valid knowledge, cannot be denied so long as a different order of reality is not realized; for unless there be an exception, the general rule prevails.
Brahma Sutra Bhasya of Sankaracharya II.ii.31
You notice how he brings in the Atman theory (bolded sentence) indirectly. What does Shankara mean? He is attacking Buddhists who think of the 'I' sense in the following manner: I .... I.... I (where the 'I' sense does not exist during the dotted time period). What Sankara is arguing is that how do these Buddhists know that the series is not I1...I2...I3 etc where I1, I2, I3 are three different ego-consciousnesses? How can there be a stable personality which remembers a unique past or recognises old friends if the ego is unstable? In fact it is these Buddhists who need an unchanging principle (the Atman) that witnesses everything for all time (i.e. even during the gaps in ego-consciousness). Only if this Atman exists can Buddhists avoid problems regarding stability of personality. Otherwise a person who is Rama at one moment will consider himself Lakshmana in the next moment after the ego comes back. If these Buddhists now say that the ego-consciousness is stable and not momentary in order to save themselves from this conundrum then they have refuted themselves. Sankara then goes on to say that he can give a similar argument refuting Buddhist realists who regard the external world to exist momentarily. Even in this case of a momentary external world you will need an Atman (an unchanging witness for all time) to give stability to our perception. So in either case you need the Atman principle to make sense of our experience.
Finally let me discuss the minor issue with Buddhism. It’s downgrading of the deities is elitist. Such an attitude is all right for the handful of people who can attain nirvana on their own. Most people are not made of the stuff of a genius where they can attain nirvana on their own. So they take help from the deities to attain nirvana. What is wrong with this? Hinduism allows persons who can attain moksha on their own to follow Jnana Yoga and also gives freedom to people who can not attain moksha on their own to take help from the deities to attain moksha. ( Whatever Buddhist theory may say, Buddhist monks do take help from the divine in practice.)
Buddhism faded out in India because Hinduism offered more than Buddhism. Moreover the mass killing of Buddhist monks and destruction of Buddhist centers of learning in Nalanda, Vikramshila etc by Muslims and mass conversion of Buddhists to Islam dealt a death blow to Buddhism in India.