When Wrath launched in 2008, the last thing I ever expected to see was a decline in the number of druid tanks. We'd just come off a successful expansion of not being Innervate bots, and I thought we'd capitalize on a new set of talents and skills in Wrath to hold our own against the brand-new death knight. Not so: The population of feral (now guardian) druids went into freefall and has never really recovered, and at the time, protection paladins took a hit too.
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.