Str gives all the tanks extra damage but Warriors are the only one of the three tanks that get massive defensive returns from the added attack power i.e. 30 seconds of Bloodbath, Inner Beast, Storms Path, Eqilibrium, Berserk + all of those skills.

DRK doesn't have as many self healing options as the warrior so I can understand a DRK opting for more VIT gear. Same goes for PLD.

If you're wondering, I use 3 Vit / 2 Slaying on my warrior and paladin.