When reading about the history of Buddhism in South Asia, the consensus is that by the time of the Muslim invasions of India (second millenium), it was already on its last legs. The Muslim invasions only helped to accelerate its decline, and the remaining Buddhists would be absorbed into the larger cultural sphere of Hinduism.

However, I have also read that during the first millenium, South India was also a thriving place for Buddhist culture. The famed monk Bodhidharma, who is said to have taught Zen Buddhism to the Chinese as well as Martial Arts in Shaolin Temple, was thought to be a Tamil Prince.

My question is, why did the Tamils eventually forget/absorb Buddhism into their Hindu Tamil practices while the neighbouring Sinhalese mostly remained Buddhist? There are Tamil communities in northern Sri Lanka, yet there are almost no Tamil Buddhists. Buddhism also virtually disappeared from Tamil lands in the Mainland South Asia. Is the Hindu vs Buddhist contrast being used by both communities as a weapon of ethnic differentiation?