Ideological and Financial Causes
The period between the 400 BCE and 1000 CE saw gains by Hinduism at the expense of Buddhism, although the evolution of Hindu ideology influenced by Buddhism was an important factor for the growth of Hinduism.
Hinduism became a more "intelligible and satisfying road to faith for many ordinary worshippers" than it had been because it now included not only an appeal to a personal god, but had also seen the development of an emotional facet with the composition of devotional hymns.
Read more about this topic: Decline Of Buddhism In India
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... After the Mongol invasions of Islamic lands across Central Asia, many Sufis also found themselves fleeing towards India and around the environs of Bengal ... In Bengal (and Kashmir, after 1323), their influence, caste attitudes towards Buddhists, previous familiarity with converting Buddhists, a lack of Buddhist political power, Hinduism's resurgence through movements such as the Advaita and the bhakti movement, all contributed to a significant realignment of beliefs that relegated Buddhism in India to the peripheries ...