I have seen majority of people having Ramcharitmanas and even temples keep many copies of Ramcharitmanas rather than Valmiki Ramayana for Akhand Ramayan Paath. Why has Ramcharitmanas of Tulsidas, which is a retelling of Ramayana, gained more popularity than Valmiki Ramayana? Tulsidas was criticized by the Sanskrit scholars of the 15th century ; is there any story why Tulsidas chose to write the Ramayan in Awadhi language. What were the events that led to the acceptance of his version more than Valmiki's?
Tulsidasji Himself laid down the threefold criterion and purpose of His composition (Manasa, 1,14,5)
It should give Him joy and satisfaction.
He must reach out to the connoisseurs and His work must earn their respact.
His work should bring benefit to the masses.
His work fulfilled all the three above.
He could put Rasa on the highest pedestal, focusing on the bhakti-rasa as the sthayi bhava. His language is very lucid and appealing to the common masses and is easy to comprehend.
So He received more popularity than the Valmiki Ramayana in the parts of India where His language is understood. In Bengal for example, the Krittivasa's Ramayana is more popular than the Valmiki Ramayana. The talent , the language and the grace of God --all of these three matter. In fact His objective was to spread the eternal message of the Ramayana for the benefit of the masses as mentioned above. He was blessed and successful.
As K.M. Panikkar puts it:
The Ramacharitamanasa is not only a magnificient epic, singing the great deeds of Rama, but a scripture of the people to which the entire Hindi knowing people..turn for spiritual sustenance; a code of ethics constantly on the lips of all, from princess to peasants and a truly fine expression of the poetic genious.(A Survey of Indian History. page 215).
According to popular legend, Tulsidas had originally planned in writing a Ramayana in Sanskrit. However, Lord Shiva appeared in his dream and instructed him to write it in Awadhi language.
In fact, in the Bhavishyottara Purana, Lord Shiva tells Mother Parvati that Valmiki will reincarnate in Kali-Yuga as Tulsidas to compose Ramayana in a vernacular language:
वाल्मीकिस्तुलसीदासः कलौ देवि भविष्यति ।
रामचन्द्रकथामेतां भाषाबद्धां करिष्यति ॥
vālmīkistulasīdāsaḥ kalau devi bhaviṣyati । rāmacandrakathāmetāṃ bhāṣābaddhāṃ kariṣyati ॥
O Goddess [Parvati]! Valmiki will become Tulsidas in the Kali Yuga, and will compose this narrative of Rama in the vernacular language. (Bhavishyottara Purana, Pratisarga Parva, 4.20)
During time of Tulsidas, Sanskrit had become a monopoly of the elite. Hence, in order to make the work more accessible to all sections of the society, Lord Shiva instructed Tulsidas to write it in popular language. However, the text isn't as popular in South India (where Kamba Ramayana is arguably more popular) as it is in North India.
It is also said that when the work was heavily criticised by elites for it being in written in popular language, Tulsidas asked them to keep all sorts of scriptures on the top of the Ramcharitmanas in the Kashi Vishwanath temple, locked, for a night. The next day, the Ramcharitmanas came on the top of all the other scriptures, even above the Valmiki Ramayana! Tulsidas claimed this miracle to be an authentication of his work by Lord Shiva. As a result, Ramcharitmanas became a more popular work, even more popular than the Valmiki Ramayana.