See, that's the thing -- we're discussing two completely different subjects here.
What groups are strong in a given area and which groups are geographically concentrated in a given area might be two different things. Like you said, Kappa, KD, Chi O and Tri Delt are consider strong Southern sororities, but they're all geographically prevalent elsewhere. Sororities that are extremely common in the South, as compared to the Midwest, East or West, might be Phi Mu, ZTA, ADPi or AOPi, possibly KD . . . and the others listed in this thread.
The same goes for any other region. For example, I would say that (from what I've seen) in the Midwest, there are a lot of very strong chapters of Kappa, Theta, Alpha Phi, Pi Phi, Gamma Phi Beta and DG, among others. However, I can't think of any sororities that stand out as particularly geographically concentrated in the Midwest except maybe Theta Phi Alpha or ASA (somebody correct me if I'm wrong here).
If you look at ZTA in a Baird's manual it becomes immediately visible how "Southern-oriented" they are -- a huge percentage of their chapters were founded in the South. They may not necessarily be as strong as Kappa Kappa Gamma is at the big universities in the South, but by numbers alone they are more concentrated in the South than elsewhere. I think this was the original topic, not which sororities have strong chapters at the big Southern universities . . . although sometimes the two topics do overlap and intertwine quite a bit.
I could be wrong though . . . that's just what I'm getting from what the original poster wrote. We all sort of took it and ran with it.