Gamemako hit the nail pretty much on the head, vit is a static buffer where as STR makes the skills that set the Warrior apart from the Paladin, whose static tanking eliminates the need for clutching defensive/self-healing during end-game. Warriors have more HP than paladins, however we're also more exposed than they are without activated abilities.

This being said, when we're using our abilities properly our stats exceed the paladin in virtually every aspect- briefly. My FC uses me as the main tank merely because I know what I'm doing, the paladin OT holds back his enmity to focus on using his additional silence that warriors don't have until he feels comfortable enough to main tank before switching over. For any other end-game team the thought of an unproven warrior tank in their pug is understandably frightening. If your rotations/macros aren't custom tailored before each fight you simply can't compare to a paladin that can 3 button spam and do everything a new warrior can't.