Buddhism began to decline on the Indian subcontinent for a variety of reasons. Some of the main factors include:
1) Brahmanical revival: After centuries of domination by Buddhist rulers, the Brahmanical religion experienced a revival in India. This was due in part to the rise of Hindu kingdoms that promoted Hinduism as the state religion, as well as the growing popularity of the Bhakti movement, which emphasized devotion to the Hindu deities. As a result, many Hindus began to see Buddhism as a threat to their own faith and began to actively oppose it.
2) Lack of political support: Buddhism had enjoyed the patronage of many powerful rulers in India, including Emperor Ashoka. However, as Hinduism began to reassert itself, Buddhist rulers became increasingly rare. Without the support of the ruling class, Buddhism struggled to maintain its influence.
3) Decline of monastic institutions: Monasticism was central to Buddhism, and the decline of monastic institutions played a major role in the religion's decline. With the loss of political support, many monasteries were forced to close or fell into disrepair, leaving the Buddhist clergy without a strong institutional base.
4) Internal divisions: Buddhism was never a monolithic religion, and over time it became increasingly fragmented. Different sects began to develop their own distinct teachings and practices, leading to a lack of cohesion within the religion.
5) Islamic invasions: Beginning in the 7th century CE, the Indian subcontinent was invaded by Muslim armies from Central Asia. While Buddhism had already been in decline by this point, the Islamic conquests dealt a final blow to the religion. Many Buddhist monasteries were destroyed, and Buddhist communities were either forced to convert to Islam or driven into exile.
Overall, the decline of Buddhism on the Indian subcontinent was a complex process that involved a variety of factors, including political, religious, and social changes.