I may have figured this out yesterday while my husband was watching "Legally Blonde."
At my chapter, we used to have an unfortunate group of PNMs that would walk in expecting the chapter to hand them a "life on a platter". It got to be really hard to talk about what they would bring to the chapter?
I think that there are still some girls coming through that are expecting to be Elle Woods right off the bat the first day. They don't necessarily see the work that it takes to run a chapter - any chapter. In a smooth, well-oiled (moneyed) "top" chapter, those things all look easier. In a chapter that is struggling, someone (or hopefully someones

) has to put a lot of work in, but can still have a lot of fun, and that chapter can do a lot that a loose group of friends cannot.
On a big campus with 16-18 big chapters, there are still niches. People from certain hometowns or high schools group here or there. Legacies stick with their legacy chapters. People in certain majors group here or there, but like KSUViolet said with that much variety - you should be able to find one where you can make a home. Plus, you should be able to leave it better than you found it.
On a smaller campus with only 6 or 7 chapters, I think the differences between the haves and have nots (or made its and working on its) are more obvious. The girls that want to walk in and be Elle - don't get the expected fairytale ending to recruitment, so they walk.
But really now - if that's all it took for them to quit, would they have made a superstar sister or a leader for the chapter?