The tanking system has long been somewhat problematic in World of Warcraft. While it scales to some degree, from 5-man dungeons to 10-man raids, the scaling falls apart when we get to 25-man raiding, which currently demands about the same amount of tanking as 10-man. You can get through most of Firelands with two tanks, no matter your raid size. Majordomo Staghelm only requires one tank, again, no matter your raid size. This means that the scaling from five to 10 works, but as soon as you go from 10 to 25, instead of needing 2.5 times more tanks, you need no more tanks.
The other problem is simply that there already aren't enough tanks for every 5-man group. When the Call to Arms feature was announced for the Dungeon Finder tool, it was created out of the simple fact that we're not seeing the distribution we'd expect in the playerbase. In order for the Dungeon Finder to work without significant group queues, we would need 20% of the people queuing up to be tanks (1 in 5 = 20%). This is not the case.
People simply don't want the perceived group responsibility of tanking. It's why changes were made to CC mechanics that allow groups to CC on the fly without pulling. It's why Call to Arms exists. And yet, despite both of these changes, tanking was still so unattractive to players that threat itself needed to be redesigned. All of this work to try and get people to tank. Maybe the problem isn't the players here, though. Maybe it's the role.