I was listening to a report that stated since the ground is made of clay the concrete basement would shift more aggressively than in "normal" soil, thus builders don't like putting them in and it compromises the structure of the home. With that said they also sell storm shelters that you can dig a hole in the ground and put a prefabricated concrete living space in. Not quite sure why everyone doesn't do that, but I can imagine that's probably pretty expensive! Disclaimer: I'm neither a builder or an engineer, I know you're shocked.