Agreed -
The big factor here is the 'small farm, older neighborhoods' factor - areas with lots that are not recent suburban (so light or no HOAs), that are not urban (antenna challenging) and that do not have massive farms / ranches / parks that eat up big chunks of the land that people might live on.
That, combined with a reasonable amount of amenities and economic opportunity is the recipe for a denser-than-average population of hams.
OH, VA, MI, MN, IL, NC, FL - these are all the kinds of places that fit this description. Some states (MD, MA, CT) are probably just as dense but have a smaller overall population so they don't stand out.
Other states like LA, MS might not have the economic engines that are behind a dense ham population, while states like NE, MT, ND, SD are just sparsely populated with massive farms and parks.
These are generalizations and of course there are exceptions. It would be interesting to run some stats to see how ham density maps to the overall state density, and my hypothesis is that a state with uniform, medium density supports the most dense ham populations