Buddhism declined in India due to three primary factors: opposition from the Hindu Brahman caste, Muslim conquest, and governmental opposition.
Since its inception, Buddhism had been considered a religious, ideological, and social threat to a large segment of the Indian Brahman caste. The Hindu Brahman ruler Shashanka of Gauda was famous for burning Buddhist images, books and relics during his reign. The Hindu ruler Pusyamitra Sunga was even more aggressively anti-Buddhist; he ordered the murder of hundreds of Buddhist monks and burned the religious texts hidden in their monasteries.
Buddhism survived in pockets up to the 13th century, when Muslim conquest from the north wiped out the remaining Buddhist communities. Islam became the dominant religion of many major cities, while Hinduism continued to be widely practiced in southern India and in the rural areas of northern India.