It's ALL about the sizes of the schools.....which = the size of their current on campus fan base, is highly correlated with their alum fan base. A Michigan grad living in Charlotte is NOT an ACC fan. Then there is the U of factor. If you drew a circle with a 15 mile radius from the top of Cameron Indoor, there'd be a lot more Heel fans in that circle. Same with SMU versus Texas, Wake Forest and UNC, Vandy and UT, and a lot of other examples.
Ironically, the largest state in the ACC is now California, and in an unusual twist, Cal does NOT have that U of cache so much, as USC and UCLA seem to suck all the oxygen out of that state. So the ACC is really on the bad luck side in the "U of" sweep stakes."