Could be any number of things really. Most likely tied to our lack of an absorption mechanic that affects special attacks, and raid utility that scales with Vengeance. Both of which Paladins and Monks have in spades. Other tanks are also less reliant on their passive mitigation than we are (Armor), and thus less gear reliant. When you're doing crazy progression like these guys are, you bend quite a few rules to get the job done.
The important thing to remember is that none of this affects anyone outside of the top 1-2% of players, at the most. Probably even less than that.