The split largely has to do with everyone having equal opportunity to play in Florida. With Florida being such a recruiting hotbed, it would have been a no-go to do a North-South split (the most obvious geographical split) because then half the teams would be down in Florida playing all the time and half would only be down there very rarely.

So instead, they basically split everything down the middle. One Florida team to each division, one SC/GA team for each division, two NC schools each, then the (at the time) two most northern schools were put in the division with FSU, and the two Virginia schools were in the division with Miami. I guess the logic there was that at least you don't have a school in Miami and a school in Boston in the same division, which would be pretty rough on traveling, particularly for the sports that don't get to fly.

Then Louisville took Maryland's spot, and they just divvied up Syracuse and Pitt into the existing divisions when they showed up.

Meanwhile, the fixed cross-division rivalry was their answer for making sure that both NC State and Duke got to play UNC twice a year in basketball, and that FSU could keep their yearly Miami game despite being in different divisions. The other "rivalries" were somewhat more arbitrary, but protecting the basketball home-and-home between Duke-UNC and NCSU-UNC was a big deal for making it all work, as was splitting up the Florida schools while preserving Miami-FSU. (I know UNC people generally don't consider the NCSU games to be nearly as big a deal as the Duke games, but they're a big deal for NC State.)

Any changes to the divisional structure in the ACC is going to be forced to accommodate those two issues. If it splits up Duke-UNC or UNC-NCSU, it'll be a no-go. If it gives half the conference more trips to Florida than the other half, it'll be a no-go.