Why's this red? Red usually has trouble killing the biggest creatures, anyway

Because my thought process was, "If it were possible for a 3cmc wrath to exist, how would I cost it?"

Making it three colors seemed most likely. Black and White can wrath on their own. Red can wrath, but usually relies on damage to do so. Black and White are being used here to mitigate red's need for damage, and Red is here because 3-color cards are allowed to be more powerful and adding red makes more sense than blue or green.