They JUST added mages to Kul Tirans so that was their chance to say "we added 1 extra class to both sides, enjoy, no favouritism here". Instead now Horde is upset they didn't get an extra class while Kul Tirans did. Hell, Kul Tirans could get a warlock or a paladin on top of it if people still thought they have too few options, and Zandalari keep their warlocks.
People are always upset if you remove stuff, if you add to both sides balance is preserved but no one feels something was taken away.
And contrary to for example class balancing where there's an issue of power creep / arms race, cosmetic options like race + class combos or number of hairstyles don't have this issue, so the more the merrier. (I still wish Nightborne would have more hairstyles, and Lightforged have more skin / hair colour options.)