It depends on exactly where you are, but generally it's due to either high water tables, the nature of the soil, or underlying rock.

- Water Tables- High water tables make leaking and flooding into the basement likely, and maintaining the outside walls of a basement against the water would be very expensive.

- Clay Soil- Clay soil is very flaky and cracky when it is dry and swells a lot when it is wet. This moving soil is very rough on subterranean infrastructure and can cause cracks in the concrete basements (which then results in flooding and destabilization)

- There is also a lot of limestone and hard rock in many parts of the south, so it is very expensive to build down into it.

Basements are also much more common in the north due to building codes that require you to dig beneath the frost line. generally, when you have already done that, there is not much additional cost (but a whole lot of benefit) in adding a full basement.