I believe most sororities must abide by anti-discrimination rules (except in the matter of gender, of course). Delta Delta Delta's anti-discrimination policy reads "The members of Delta Delta Delta, assembled in Convention for 50th time in the history of the organization, acknowledge and affirm that Delta Delta Delta, in its selection of members, does not discriminate on any basis other than gender, and expressly will not discriminate on the basis of ethnic heritage, national origin, personal appearance, personal beliefs, race, religion, or sexual orientation."
Does that mean that it doesn't happen? Of course not. I'd be willing to bet that at least one of those criteria has been used as justification for cutting during rush at every Tri Delta chapter around the nation.
So why don't more people sue? Well, first of all, I think the most common reason to be cut is the "personal appearance" reason, and most people have more pride than to file a lawsuit saying "Tri Delta cut me because I'm ugly!"
(And who even knows how a lawsuit like that would hold up in court!) Furthermore, in most cases there's no proof. Melody Twilley (black rushee at Bama, which's NPC system is basically entirely white) can speculate all she wants that the reason she was cut from every sorority during formal rush is because she was black, but she has no proof for it. The sisters can just smile and say sweetly, "We just didn't find that she fit in with our chapter." I think that the UGA case is one of the few where there was some sort of "proof" that race was one of the deciding factors that this girl wasn't initiated.