I believe it's the beginning of the Great Plains and generally lacks water. It's not dry like a desert, but compared to the temperate forests and rivers of the East, South, and Mid-West, it is. It is intensively farmed now, using pumped aquifer water (https://en.wikipedia.org/wiki/Ogallala_Aquifer), but it never developed the population density of the East. Geology maps also show this divide, so it is based on the underlying rocks, as well as weather patterns from the Rocky Mountains I suppose.