My best guess remains that the popularity of the death knight and the protection warrior in early Wrath pushed a lot of druids and paladins out of tanking. We had more role options than they did, so respeccing to melee, heals, or ranged DPS was a better option than getting yourself or others benched. There were other things going on that probably didn't help much, but at the end of the day it was a numbers game that the bear and paladin simply lost. It was a vivid lesson that design decisions that don't necessarily have much to do with your class or role can wind up having a serious impact on them anyway.
Filed under: Analysis / Opinion, Raiding, Warlords of Draenor