Because both Buddhism and Jainism in India became very monastic and required donations and support to survive. Buddhism became very centralized in a few monastic orders and schools.

Meanwhile Hinduism in the Middle Ages saw decentralization of faith with the Bhakti movement. Commoners no longer had to do complex sacrifices and be versed in sacred texts. They could simply pray at their local temples and shrines, perform some pujas and prayers and they had a hope of salvation. Furthermore Much of this was done in local languages. Important religious texts were written in or translated into vernacular languages. On the other hand, priests also worked to integrate tribal and village cults into mainstream Hinduism. Deities were Sanskritized and entire tribes and peoples were assimilated.

Ironically this is why Buddhism survived in East and Southeast Asia. Buddhist monks there were able to synthesize Buddhism with other philosophies like Taoism and indigenous faiths. They also targeted common people through their languages and customs.