[cstejerean]

The map might be a bit misleading. In areas like Chicago there are a handful of giant WalMarts serving millions of people. In areas with lower population density there tend to be more stores (and less people). Take a look at the map of population density by state:

http://en.wikipedia.org/wiki/Image:USA_states_population_den...

The fact that WalMart originated in Arkansas skews the numbers for the south, but for the rest of the country there is a clear correlation between population density and stores per capita. Notice how California (12), Illinois (11), Ohio(9), all have lower per capita WalMarts than North Dakota (47), South Dakota (46), Montana (48), Wyoming (49).

The numbers in parentheses represents the state's place in terms of population density.