Right, and places like Texas, Colorado, Oklahoma, and Montana are not included in the Midwest. The point isn't that the wind belt region makes up exactly 1/5 of the population, but that there are many people there that shouldn't be dismissed because they aren't in the most densely populated cities.
The wind farms in SW Minnesota/the eastern Dakotas/ NW Iowa (http://www.nrel.gov/gis/images/30m_US_Wind.jpg) are relatively close to the larger urban areas in the region and I suspect some of it might even feed into Chicago.
There's not much overlap between the wind belt and where people live. But there are many round farms in the wind belt. The unused corners of round farms would be good places for windmills. There's road access in place. Extra revenue for farmers. Won't bother the crops.
Collecting up the power and shipping it somewhere useful is a problem. That requires new high voltage transmission lines and collection at substations for conversion to DC and long distance transmission.
But the Midwest does not completely overlap the wind belt. The more densely populated parts of the Midwest are especially far away.