There has to be an ingrained practice that nobody dared to change.
The Mughals din bother messing with it.
The muslims neither.
Nor did the Sikhs
The British did not attempt to meddle with it, they simply called the untouchables who embraced christianity as Caste Christians.
The Kerala Communists steered clear of the issue.
Untouchables provide service to the other castes.