What was the reason behind the fall of Buddhism in India? Buddhism was originated from India.It spread all accross the world, many countries are called as Buddhist countries. 

Lord Buddha was the most scientific, serene, logical and kind person of the world, anyone becoming his enemy seems to be impossible. There was a time when almost whole of the India was Buddhist. Then, what caused the decline of Buddhism in India?
After the Pari-nibbhana of The Buddha, there was a decline of Theravada Buddhism ( those who follow the teaching preserved in the Pāli), and a rise of the Mahayana Buddhism. The third council got underway to make the monastic order of those who follow the teaching preserved in the Pāli.  With time Mahayana Buddhism rose up again. It was a death blow to those who wanted to keep the teaching preserved in the Pāli.  Mahayanists translated the Dhamma into Sanskrit and rejected attaining of the Marga- phala of the Theravada School. 

At the time only the Brahmins’ Veda was written in Veda and Sanskrit languages. Brahmins never felt that other religions will be a threat to their own since Veda as it was the only religious script written in Sanskrit. However, after the translation of the Mahayana scriptures into Sanskrit by the Mahayanists, the Brahmins started reading these translated texts and to argue pointing out its content. 

In trying to defeat these Brahmins, the Mahayanists built large universities like Nalanda with the aim of learning the Veda. The language used in these universities was Sanskrit. They tried to outdo the Brahmins. They tried to argue with the Brahmins. Thus what came to pass was exactly as mentioned in the Alagaddūpama Sutta, which states that “if anyone tries to learn my teachings to argue with others, then he will be destroyed like the man who tried to catch a snake by its tail.”

When the monks won an argument the monks got the king to make the Brahmins their servants in the Nalanda University. The monks were beheaded when Brahmins won their arguments. The Brahmins went for the kill, because of the change that these monks have made to the Brahmin society. A Sothern Indian Brahmin called Sankarāchārya rose up to destroy the power of these monks. The Musālman invaded India at this time. The Brahmins got them to first destroy Buddhism, or at least what was left of it. This was how the Buddhism got wiped out from India. 

It was the moguls that did most of the killing of Buddhist Monks, and the process of decline continued until Buddhism got fully wiped out in India by the end of the 12th century, when Somapura Mahavihara (in the present day Bangladesh) was set ablaze. In 1199 CE Odantapuri Mahavihara close to Nalanda was burnt down after killing all the monks. Buddha’s place of Enlightenment, Bodhgaya too was attacked.