Guys (and women who really understand men)please help me break this down for my friend?

She wants to know why guys check her out on the street but don't ask her out. And why bums/homeless people always hit on her and not the good looking guys she is into. I have walk down the street with her and have noticed that she gets noticed - but can't really put my finger on why she dosen't get asked out more. She thinks it is b/c she is tall and lean I think she's cute- not glamour girl- but people tell her she should model every now and then). help me help her - don't know what to say.