Why are tornadoes severe in tornado ally?

A number of climatic factors come together in the region known as Tornado Alley.

Warm moist air from the Gulf of Mexico moves over the region, carrying enormous amounts of energy to fuel thunderstorms. It can then collide with cool air from Canada (along a cold front) or dry air from the Rockies (along a dry line). This releases the energy carried by the warm, moist air producing thunderstorms that can become very strong. They can be especially violent where a dry line intersects a cold front. Additionally, Tornado Alley gets a lot of strong wind shear. This can intensify the storms even further and can also give the the rotation they need to produce tornadoes and generally speaking the stronger the storm, the stronger the tornado it can produce.