I think you have the reason backwards, somehow Overwatch as a game had mass appeal and that lead to the deluge of character porn. I bet there's tons of porn of that disgusting creature I posted earlier too just because Overwatch is huge. Almost anything popular gets a flood of porn, though I suppose people do like the characters for some reason too.
Still, the design is C+ at best, even the most generic anime girl is more appealing (both as a cuter design and a more interesting character to fight as, look at her suit, it's so boring).