Traditions that converged
By the 8th century Buddhist and Brahmanical thinkers shared a great deal of vocabulary and argument. Tantric practice developed on both sides, in forms close enough that scholars still debate who borrowed what. Vedānta philosophers absorbed Buddhist reasoning while rejecting its conclusions.
At the same time the devotional movements that became Hinduism offered something Buddhism had largely left to monks: a god one could love, worship directly, and reach without renouncing the world.
Absorbed rather than defeated
Purāṇic texts came to list the Buddha as an incarnation of Viṣṇu. Shrines kept their worship with the Buddha understood as a Hindu deity. For a lay Buddhist, the boundary became hard to feel: the festivals, the life rituals and the neighbours were much the same.
Buddhism in India also had no household priesthood of its own. Brahmins performed weddings, namings and funerals for everyone, Buddhists included. So when the monasteries weakened there was no domestic structure to carry the tradition on. In Sri Lanka and Southeast Asia, where the monastic order was woven into village life, it survived every bit as much upheaval.
Why it matters
It shows the decline was gradual and religious, not only political.
History and tradition
What historians can say
A cause by cause account is not possible: the sources are thin and mostly indirect. Scholars list conversion, the loss of patronage, invasion, the concentration of learning in a few institutions and the fading of a distinct lay identity, and still argue about the weight of each. Buddhism also never disappeared entirely from the subcontinent: it continued in the Himalayan belt, among the Newars of Nepal, and among the Barua communities of Bengal.