Conventional wisdom would suggest
I'm sorry, but what conventional wisdom? Urban and suburban convictions of their own surpreme splendidness?
A lot of rural places have a surplus of jobs compared to their population due to natural resources. Housing and living costs are a lot cheaper so you get a lot further on a smaller income than you would in a big city. A family in the rural Midwest can live a comfortable middle-class life style on the same income that would leave a family in New York barely scrapping by.
As to why Southern/Appalachian rural areas are horribly off compared to the Midwest, there are a lot of historic, demographic, and political reasons for that.