Yeah, I wonder. It does seem that vampires wind up dominant over shapeshifters, and that might have something to do with it. Also the fact that it seems much more frequent (to me) that vampires are coded sexy: it's really not infrequent in my experience that people become more attractive when they get vampired in books (explicitly so - pores closing up, skin tone smoothing out, losing weight, etc), but not so much so when they get werewolfed; the bite of a vampire is often coded in sensual or pleasurable terms, whereas the bite of a werewolf is much more often pursued-by-ravening-hellbeast and not very pleasant even when the werewolf is a good person.

Of course, it's sort of chicken-and-egg. Do vampires win out because they're sexier (or more dominant), or are they allowed to be sexier (or more dominant) because they're going to wind up with the protagonist?