Why do people worship Krishna?

"Because they believe in a fake God who even doesn't exist. There is only ONE GOD"

This was a previous answer, written by someone who obviously hasn't done his/her/it's(?) research. Krishna is an Avatar (incarnation, physical manifestation) of the deity Vishnu - there is only one god (Brahman) in Hinduism, and the deities represent diffferent aspects of that one, eternal, formless ultimate reality. As for your question, Krishna is widely worshipped as he is regarded by Hindus as a leader, hero, protector, philosopher, teacher and friend all at the same time, and an ideal deity to look up to - he has influenced the Hindu thought in countless ways.