>>20319646>>20315498Gamefreak probably thought Fairies only resisting Fighting wasn't enough to completely balance the type's dominance. So what Fairies resisted their STAB? They won't stop Fighting types to set up in front of them and hit fairies with boosted SE coverage moves. So they make it SE against type to further balance it.
I sorta approve of this decision tho. Not only Fairy is great coverage on Fighting-weak mons that are dead weight weight against Figthing types, it also made the current fairies that were once weak to Fighting(Clefable,Granbull,Mawile,etc) more viable niche-wise while flipping the tables with Fighting types by destroying them super-effectively for the sake of balance and revenge.