For whatever reason; aesthetics, mechanics, lore, etc.
Personally, I really can't stand Mages. I've been levelling one via Recruit-a-Friend lately, and everything about the class beyond Portals/Teleports just makes me want to hate it. Arcane spec is far too predictable/boring, Fire is too proc-dependent, and Frost just feels incredibly clunky with all the instant cast procs coinciding with cast time spells. If there was any class i'd want to get a MoP-Warlock-style revamp (i.e. Completely redone from the ground up), it'd have to be Mage. Don't even get me started on the level 90 talents; you're either channelling evocation once every minute or so, forever stuck to your runes on the ground, or throwing up a shield in the hope that you take some damage in order to actually get the buff.