I don't think that's even a secret. Even last mauryan emperor brihadatha became Buddhist and gave privileges to Buddhist monks who opposed Hinduism and Hinduism was on decline in 2nd BCE so the pushyamitra shunga, emperor's general killed him and became emperor and then Buddhism declined in India. If it weren't for pushyamitra, India would be a Buddhist majority country today. They even teach us this in school.
So Buddhism was already near extinction in India upto 12th CE and bakhtiyar khilji's invasion and sacking of Nalanda University which was the biggest centre of learning of Buddhism finished and marked the end of remaining of Buddhism in India.