Because midwest was a word before most of the states existed, and "the South" is a cultural term just as much as it is a geographic one. When we're talking about "Southern" states, we're not talking about states in the geographical South most of the time. It's basically the states that were a part of the CSA during the American Civil War.
Since I live in the middle of the Great Lakes, I found this one easy (just go for nearest neighbors and spread out from them). I was surprised that states I'd consider part of the great plains were included, but perhaps the distinction is based on agriculture. In Kansas and the Dakotas, crops outweigh livestock; west of these, ranching dominates. Just a tentative thought.
south Dakota, north Dakota, nebraska, and kansas aren't really midwest. Midwest is generally considered to be the great lakes, missouri, and maybe western pennsylvania.
Also try Midwestern States in Order of Size