Why are there more tornado-producting storms in flat plains than in mountainous areas?

The two together actually cause the tornado. The cold air descending from the mountains heats up with the warm air coming from the plains or low lands, causing wind streams and inclement weather.

Although they can be in either i'm pretty sure they just meet in the middle.

Storm-producing storms can be formed on plains or mountainous areas, depending on where the two different temperatures / humidity meet to form thunder clouds. In the United States, the dry line or the place where the moist winds move north into the spring and summer of the Gulf of Mexico, find the driest, driest winds moving to southern Canada on the Central Plains, which is why most US tornado storms come from flat plains.