what makes a sorority "lower tier?"

I feel like this is almost always based on looks. All of the sororities are more or less equally involved on campus. All of them have mixed bags of girls in terms of personality. So the only thing I can think of really is looks.

As in, the more "slam pieces" they have, the higher they "rank". Sure, there seem to be other factors involved like how many competitions they win and how many mixers they have, but those don't factor in as highly from what I can see.

Correct me if I'm wrong? What makes a sorority "bottom tier?"