The conventional wisdom is that, in general, VCs join the boards of the companies they invest heavily in. Which means they'll have to travel to attend a monthly board meeting. And it's easier to only invest in companies where they don't have to travel too far to get to the monthly meetings. Of course, I think it's a terrible excuse, but that partially explains things.

edit: Bryce Roberts at OATV is currently running an experiment that could challenge that default VC mindset with Indie.vc: http://indie.vc/