>>37505339Mawile was just a "it's there" Pokemon. Nobody ever used it in competitive. Its base stats were too low, its movepool was shit, and other Steel-types outclassed it. Aside from it being the only pure Steel that wasn't a legendary, Mawile had no real calling call.
Gen 6 made Mawile better by giving it one of the best defensive type combinations in the game, as well as a pretty good offensive STAB overall. And this is not even factoring Mega Mawile, one of the best Megas in the entire game, that has everything Mawile had and more.
>>37509908Mawile has steel jaws. It was also the version-exclusive counterpart of Sableye, a Dark/Ghost Pokemon, so it didn't make sense for Mawile to be a pure Dark-type.