It has to do with a concept called "falling upwards" -- see the Wikipedia entry below:
Wiki entry on Falling UpwardsEssentially, we go for someone we deem to be slightly better quality than ourselves. So if the guy isn't respecting them, clearly he's better quality, right?

Guys also do this -- I'm sure we've all passed on the homely or fat girl (or just unattractive to us) who was nice and valued us for who we are, while we pined for the bitchy pretty girl who wouldnt' give us the time of day. It's just nature at work.