[reg45]

So, if the bordering states of Indiana and Illinois are midwestern, what does that make Kentucky?

[John Milner]

So Tennessee borders Kentucky. If Kentucky has to be Midwestern because it borders Illinois and Indiana, then Tennessee has to be Midwestern too. Oops, there goes Mississippi and Alabama.

Basically they have to draw the line somewhere. They chose the Ohio River as the divider between Midwest and South. Illinois and Indiana are on the North shore. Kentucky is on the South shore.