They are so OP that they are were incapable of tanking 2 bosses on heroic progression (Gara'jal and Ra-den).
Noone is saying "Dk's aren't any good". They are just the least desirable right now. All tanks are good enough to tank heroic progression but if I had to choose out of the 5 tanks Druids and DKs would be the last 2 but Dks would EASILY be last.