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

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.

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.

Europeans are more concentrated in urban areas.

Humid continental climate is found in the interior of continents between the 30 and 60 degrees latitude, but usually above the 40 degree line.   40 degrees north of the equator means less direct sunlight and therefore less warmth.   this climate is mainly found in the northern hemisphere since there are no major landmasses in the southern hemisphere.  if you live in northern indiana, you know all about humid continental because that is the climate type you have!