The reason, as mentioned a coupled of times, is mostly just historical. Paladins and druids have typically been very easy to tank to say 80% of their capacity with, where a warrior has required more interactivity to get that same level of threat generation.

I'm not sure where it stands now. Paladin tanking is more interactive, but still fairly simple if you don't need optimal agro generation (you don't). I havent' even leveled my war to 85 yet, but he still seems a little more needy to setup a basic rotation.

So I'm pretty sure it's mostly just because that's what we all have from the past