Because that is where they are most likely appear due too the land around they usually land on more of a flat terrain than a hilly one...Hope this helps.
Numerous tornadoes occur in Tornado Alley because of the contrasting air masses that tend to meet in the middle and collide, producing strong and violent thunderstorms. The region of Tornado Alley is an excellent environment for the recipe of these severe storms, containing all the ingredients for tornadic conditions such as dry air and temperature inversion. When dry air is guided northward, it remains elevated to which creates temperature inversions over this region along with advancing moist, creating unstable air. The temperature inversion stops the rising, warm air, hence when you start to get upper level winds. These winds can cause the inversion to shatter, all the energy this inversion was holding being released into severe thunderstorms with no doubt of tornadoes.
