It’s all the middle junk nobody wants which is why there’s so much of it.
Seriously tho, the historic context is that “The West” was west of the Mississippi river, which iirc got started calling that after the Louisiana Purchase. So the previous “west” wasn’t accurate enough so the states between the Mississippi and the Appalachian mountains (chiefly the northern states around the Great lakes). Then the area has just kind of expanded to include the plains states since they’re also flyover country