Because caste is scripturally fixed, reason cannot reform it.

Evidence

It is therefore clear that in any matter on which the shrutis and smritis have given a positive direction, a Hindu is not free to use his reasoning faculty. The same rule is laid down in the Mahabharata: … He must abide by their directions. Caste and varna are matters which are dealt with by the Vedas and the smritis, and consequently, appeal to reason can have no effect on a Hindu. — Ambedkar, B R, Annihilation of Caste_ The Annotated…, 982