They left one out
If they would have added a fourth map for snow events, the overlapping of the four maps would have virtually covered the entire country. The types of tornadoes we've seen this year are rare. Major snow events don't kill hundreds, but they kill dozens and they occur every winter.
Perception is an amazing thing. They seem to de-emphasize the risk associated with earthquakes, whereas I, who live in the Houston area, am not especially afraid of hurricanes.