Why do most tornadoes in the US happen in tornado alley?

Tornado Alley sees a high frequency of tornadoes because of their ideal climatic setup. In the spring storm systems cause warm, moist air from the Gulf of Mexico to collide with cool air from Canada and dry air from the Rockies, resulting in the formation of strong thunderstorms. About a mile off the ground a layer of stable air holds thunderstorm activity back, allowing instability to build underneath it. As a result when the storm break through the cap the instability will be enormous, leading to explosively powerful thunderstorms. At the same time wind speed and direction varies with altitude, which sets these storms rotating and also strengthens the updraft. This rotation combined with strong thunderstorm updrafts can then produce tornadoes.

Such a setup does not occur as often in other parts of the U.S.