Heavy metals in the nearby earth are the obvious parameter here--you need something to decay into radon for there to be radon.

But I think another factor has to do with the water table. In some places the water table rises high enough to flood basements, so you don't usually have basements at all in those places, and subsequently you don't have large reservoirs of radon adjacent to living spaces. I know of some people here in CO (where we have decaying granite in the soil) that don't have a basement and still need a radon ventilator, but it's not very common.