They are often rain wrapped. Since the weather in the southern states is more temperate, the homes are built to different specifications than those in states that experience colder weather. Many homes are set on blocks rather than on a slab foundation or over a basement. They may be built of unreinforced cement or cinder blocks. A high water table in some states precludes construction of a basement.
Rural poverty is prevalent in some areas.
These factors make the homes and their occupants in the Southern states more vulnerable.