When you're tanking its very easy for your pet to start attacking the wrong mob that should be CC'ed, or to star hitting a mob while its being pulled if you have to pull. Yes you should put your pet on passive and make it stay at camp and only attack on your command, but at that point you're losing a lot of the point of an autonomous pet if you are micromanaging ever action.
Its much harder for a tank to manage a pet as a tank when you should be focusing on your own survivability and maintaining threat.
There's a reason there has never been a tanking class with a pet in any other game.
