For starters, because characters already had very high crit numbers in the first tier of content.
So if characters already have too high crit percentages why the hell do they make it so we get Crit Rating from Spirit and nerf it to the ground every single Content Patch because "characters have very high crit numbers" ?
Are they too proud to just give us spelldamage from spirit? Every other class gets Spell Power from Spirit or Intellect and everything is fine with balancing... come on, they should jump over their shadow and just do it.
At all the PvP talents they are preaching "less RNG", "RNG can screw it all" and for PvE they just give us even more RNG. Crit Chance is RNG and the T8 4piece Bonus is even more RNG. I don't want one fight 7k DPS because of holy proc luck and the other week 4k because the RNG just doesn't like me, I wan't the damage when I need it. And when I need it I use combustion as Fire Mage and there I get nothing from Crit Rating on Molten Armor.