Skullgirls' engine, a.k.a. the Z-Engine, was primarily made by a certain Mike Zaimont, who ended up being not only a chronic sexual harasser, but also an exploitative boss to his employees. So much so, that the development of Skullgirls was eventually moved out of Zaimont's company, Lab Zero Games, into a new company formed by his former employees, Future Club Games. The only problem being that Future Club still uses the Z-Engine, apparently because reimplementing the entire game in a different engine is economically unfeasible (unlike, apparently, going through all of the artwork of the game with a fine comb to make it less Nazi and less sexualized). Same goes for another game to which Lab Zero licensed the engine to, Them's Fighting Herds (a.k.a. the offbrand MLP fighter).
reimplementing the entire game in a new engine is economically unfeasible
This is equivalent to literally making the game from scratch and changing some art assets is no where near as complex and time-consuming as that would be.
