Not the Vamps its the Dawnguard, the Vampire Hunters, that use them to slay vampire and thus the vampires despise this weapon. Vampires usually use their knowledge of lost magic to do battle.
Apologies, it is to cold and too early for me to think then, I retire from the argument. And no I am not ruining my DLC experience by checking it out that closely before I play it, so I better make a run for it.
But since the Dawnguard know about the vampire group and hunt them, maybe they actually went and investigated the Dwemer weapons and other ancient examples of weaponry and decided a crossbow would suit their purpose better than anything native to their culture?