As much as Gautama Buddha influenced history and the ideas have spread across SE Asia, mostly, Buddhism didn’t persist in India because the Hindu caste system was stronger than his (or a cumulative “his”) ideas of nirvana, or “release from cycle of rebirth,” which directly challenged the Hindu social order.
It was up to the Europeans to figure out the concept of human equality, and the Americans to make it happen.
... based on Christianity, I should have added.