If you look at where some of the NPC sororities were founded (in Illinois) and where most of the Big 10 schools are, it makes sense that there is a strong concentration of single letter chapters. Kappa, Pi Phi and Alpha Xi Delta, for instance are all within about 4 hours of 6 of the Big 10 schools. And just a bit further than that to Ohio State. So it makes sense that they expanded in concentric circles.

:D