With MoP came a change to the traditional gemming system; You obtain less primary stats per point from gems than you do secondary stats. The secondary stats on gems are double the amount of primary stats, which makes secondary stat gems(specifically crit) that much more valuable. This does not mean that strength is useless to us, because that is far from true. All it means is that when it comes to gemming, crit takes precedence because you can, point for point, obtain more value from it.
Crit is being valued so high now because both Bloodthirst and Colossus Smash critical strikes proc the Enrage buff and this is a major component of our DPS as fury because it enables the use of Raging Blow, as well as giving us increased damage while it is active. Bloodthirst has twice the critical strike chance and essentially we're aiming to get almost guaranteed enrages from BT.