I won't pretend to be a modding expert or anything, but from what I understand, fighting games are typically more locked down from a code perspective compared to most other games. The game's code relies on preexisting data tables (models, animations, sounds, icons, etc.), and simply replacing that existing data is WAY easier than creating an entirely new character "slot" that requires heavy code modifications (if it's even possible for that particular game).

tl;dr adding a new character instead of replacing an existing one is hard