With Burning Crusade, paladin tanks became not only viable, but also prefered due to their synergy with casters, where they benefited from spell power and could provide mana buffs.

They've since balanced away from that, but they still maintain a a special spot in people's minds as a hit-it-and-forget-it tanking class.

Of course, this lead to further rebalancing of other classes, and at this point, Warrior is a very powerful tank I would rank up there with Paladins. They have plenty of AoE threat now, arguably better so due to thunderclap sharing rend and cleave being cleave.

I think the only reason you don't see more warrior tanks boxing is that people generally don't try new things in preference to the tried and true formulas. But Warrior tanks work and they work very well, so if that's what you want to run with, go for it.