The midwest is huge, but I'm going to assume you live in a humid subtropical/continental climate (most of the midwest). This means that the weather is subject to extremely cold air masses from the far north, and warm, humid air masses from the Gulf of Mexico. The fact that summers are more humid than winter means that temperatures are more stable as well (water in the air does not allow volatile changes), winter is cold and dry, giving room for drastic changes.

By the way, the midwest has gotten warmer in the past years and the trend suggests that temps will continue to rise.

TLDR: humidity = stable temps