Specialized Tanking

Requiring certain types of tanks to do certain fights may be fun for some, but is bad for the game overall.

Even if you could somehow make certain that paladins were the best at some kind of bosses (so that they didn't just become the trash tank), you are then tied to make sure every raid has a few of those types of bosses. Ulduar becomes 3 warriors bosses, 3 paladin bosses, 4 druid bosses, 4 DK bosses. If one of those bosses is perceived as more difficult, more necessary or rewarding of better loot, then the sense of inequity still exists.

Furthermore, we don't want every guild to have to keep 4 good tanks on their permanent roster and just slide them in for the right fight. While dual-spec makes that slightly easier than before, we still don't think it's fair to demand every guild keep someone on hand who can spec and gear as a Feral druid or Prot paladin.

We're fine with certain tanks having a slight advantage on some fights, because we recognize that's probably inevitable as long as the abilities are different. This is going to always be trickier for the hard modes, but again the goal is that you can have any class of tank as your main tanks.
Source