You are making a naive assumption here that there was no religion present in India before Buddha, and that after Buddha established his Dharma everyone should have abandoned their respective religions to take up Buddhism.

Ashoka converted to Buddhism and ruled almost the whole of the Indian subcontinent at one point. But that doesn't mean he converted all Hindus to Buddhist. Few Hindus may have converted, but 'land of Buddha' also happens to the 'land of Sanatana Dharma' where there are several other deities people worshiped and traditions people practiced.