To keep it out of the way while fighting and feeding. It's bad enough trying to get the victim's hair out of the way while feeding so you add trying to keep your own hair out of the way, too? It's too much. Plus, have you ever try to get dried blood out of hair? It's a nightmare.

I always assumed vampires would just gather together and lick each others hair clean, I mean why else would vampires hang out with each other? Too play baseball???

Of course not...they play cricket.

...and: