I believe the problem in vampires lies in the range of roles they have taken before. They've been the hero, the antihero, the arch enemy, the evil minions, the mysterious neutral, the glittering retard thing, and countless others. Whatever you portray a vampire as, someone reasonably well known has probably done it before. Werewolves, on the other hand, almost always appear in the same one or two clearly defined roles, so have scope to be developed, adapted and changed to make them seem fresh.
Alternatively, perhaps I'm wrong; maybe vampires are easier to write about if you're a less able writer, resulting in larger quantities of lesser quality about them and we're just tired of it piling up?