People worship Krishna more than Vishnu (I guess so, because heard more about Krishna than Vishnu).
Yes, you are right. You must have seen ISKCONaites and others (non ISKCONaits) who worship Him. But there are followers of Lord Vishnu as well who are named as Sri Vaishnavas. One can find them mostly in South India.
Does it mean Krishna is still alive? Or people worship him for other reasons?
To answer it, let me categorize it into three groups of people.
The people who believe Lord Krishna as Brahman:
In this group, people consider Lord Krishna as Supreme one who appeared for his devotees and disappeared as per His will.
From Srimad Bhagavatam, The Disappearance of Lord Sri Krishna 11.31 :
SB 11.31.5 — Seeing before Him Brahmā, the grandfather of the universe, along with the other demigods, who are all His personal and powerful expansions, the Almighty Lord closed His lotus eyes, fixing His mind within Himself, the Supreme Personality of Godhead.
SB 11.31.6 — Without employing the mystic āgneyī meditation to burn up His transcendental body, which is the all-attractive resting place of all the worlds and the object of all contemplation and meditation, Lord Kṛṣṇa entered into His own abode.
SB 11.31.8 — Most of the demigods and other higher beings led by Brahmā could not see Lord Kṛṣṇa as He was entering His own abode, since He did not reveal His movements. But some of them did catch sight of Him, and they were extremely amazed.
SB 11.31.9 — Just as ordinary men cannot ascertain the path of a lightning bolt as it leaves a cloud, the demigods could not trace out the movements of Lord Kṛṣṇa as He returned to His abode.
SB 11.31.10 — A few of the demigods, however — notably Lord Brahmā and Lord Śiva — could ascertain how the Lord’s mystic power was working, and thus they became astonished. All the demigods praised the Lord’s mystic power and then returned to their own planets.
SB 11.31.11 — My dear King, you should understand that the Supreme Lord’s appearance and disappearance, which resemble those of embodied conditioned souls, are actually a show enacted by His illusory energy, just like the performance of an actor. After creating this universe He enters into it, plays within it for some time, and at last winds it up. Then the Lord remains situated in His own transcendental glory, having ceased from the functions of cosmic manifestation.
SB 11.31.12 — Lord Kṛṣṇa brought the son of His guru back from the planet of the lord of death in the boy’s selfsame body, and as the ultimate giver of protection He saved you also when you were burned by the brahmāstra of Aśvatthāmā. He conquered in battle even Lord Śiva, who deals death to the agents of death, and He sent the hunter Jarā directly to Vaikuṇṭha in his human body. How could such a personality be unable to protect His own Self?
SB 11.31.13 — Although Lord Kṛṣṇa, being the possessor of infinite powers, is the only cause of the creation, maintenance and destruction of innumerable living beings, He simply did not desire to keep His body in this world any longer. Thus He revealed the destination of those fixed in the self and demonstrated that this mortal world is of no intrinsic value.
From the above verses, it is clear that Lord Krishna left this world and moved to His eternal abode (Goloka), which is considered as the Highest planet by them.
Lord Krishna is unborn, hence there is no point of death which He (Lord Krishna) confirmed in Bhagavad Geeta:
I am never manifest to the foolish and unintelligent. For them I am covered by My internal potency, and therefore they do not know that I am unborn and infallible. BG 7.25
Those who believe Lord Krishna as an incarnation of Lord Vishnu:
This group of people quotes a verse from Bhagavad Geeta which states that Lord Vishnu appears in every millennium to establish dharma BG 4.7
Those who do not consider Lord Krishna as God:
This group of people uses words like "birth and death" kind of words for Him.
Now, one question may arise if Lord Krishna "appeared and disappeared" then how was his body made of. For this, please refer to my answer here and this answer by @Hashable