I can only assume its because the writers know the audience already knows about vampires*...whereas in things like Supernatural, they literally have to spell out that the Fates are Greek gods.

that and society seems to be in a "vampires are cool - as heroes or villains" phase still.

* = at least Mr. Butcher was inventive when it came to his vampires (the inner tube skin, among other details). too many writers are lazy and just put "there are vampires."