They were meant for mages, who can't dual wield and can't shield block, so they really have nothing in their other hand anyways, so it doesn't matter that its 2 handed. And since parry is the only thing that mages can do to stop an attack, the extra parrying abilities that you get with staffs are nice.
Besides, when you think of a warrior, you never think of them holding a wimpy staff. They have a giant bloody axe or sword or something. But when you think of a wizard the first thing that comes to mind is an old man dressed in robes, carrying a staff.