After Buddhism spread to East Asia why did it vanish from India and Hinduism returned?
Answers
All religion was/is dependent on state support. Europe became Christian because of rulers became Christian. Same with Islam.
Buddhism was the state religion of many great empires in India - Ashoka, Kanishka, Baladitya, Harsha Vardhana and Pala dynasties and I am sure many others as well.
However, it was sadly not supported by the new kings that came into India during the Middle Ages who used the brahmanical caste system to consolidate their power. They made up fictional lineages to seem superior to others and spread the Ramayana and Mahabharata, which is why these texts are still major in India.
Many of these new kings were hun descendants who despised Buddhists since The Buddhist Gupta King Baladitya defeated the huns thoroughly so they especially did not support Buddhism.
Almost immediately afterwards Islam came and used this same caste system and made it so oppressive that many converted to Islam.
Explanation:
The decline of Buddhism has been attributed to various factors, especially the regionalisation of India after the end of the Gupta Empire (320–650 CE), which led to the loss of patronage and donations as Indian dynasties turned to the services of Hindu Brahmins.