That corridor through the US has low population density, had little or no manufacturing, and was always agricultural/ranching country. I'm sure that has something to do with the job stability there. No economic bubbles there, so they don't have bubbles deflating now.
Excellent graphical picture of unemployment. Good find. It looks like there is one county in the Dakotas that has under 2 percent unemployment. Maybe we should all move.
"It don't make no sense that common sense don't make no sense no more"
John Prine