The type of ground in the area is red clay, which is extremely hard to dig through. This makes it very very costly to dig basements, especially for large buildings like apartments and schools.
Sad as it is, the cost is so high as to be prohibitive when it comes to tornado emergency shelters. Tornadoes are rare, short-lived, and almost never as destructive as the one from yesterday. Most large buildings are built to withstand up to an EF3, which protects against most tornadoes.
Another point is basements are (sort of) a requirement in cold climates such as your own, or here in Canada, where the foundation needs to be below the frost line to avoid having the building shift and eventually collapse as the ground freezes and thaws over the years. Since you have to dig that deep anyway, there's no reason not to use the space you're excavating as an extra floor below the ground level - a basement.
In warmer climates, a basement is entirely optional. You can save a lot of money by skipping it, and also some potential annoyance with flooding as well. So it's a lot less common.
