The main problem is that mages lack group utility for heroics. Basically, all you get is CC, long-cooldown spell interrupts, +/- magic dmg and remove curse. Consider the wealth of utility you get with shaman setups or multiclass setups. Resists, anti-fear, auto-poison/disease-cleanse, wipe-prevention, AoE heals, full-spectrum resists, high-utility/dmg pets, taunt pets, speed, all sorts of good things.

Back in the day, there were a few boxers that ran mage setups. Just not so many these days because shaman groups have proven so much more effective in a wide range of situations.