Why is it that tornadoes occur in some parts of the country more than others?

Tornadoes depend on a number of factors to form including instability (which involves warm, moist air developing or moving in under cool air), a system to trigger thunderstorms, and wind shear to give storms the rotation they need to produce tornadoes.

Due to the locations of mountains, plains, warm and cold ocean currents and semipermenant high and low pressure systems the necessary conditions are more common in some regions than others. Some areas may see a high incidence of tornadoes while others may be too cold, too dry, have too little wind shear, or too little temperature variation to see large numbers of tornadoes.