Here it is a transportation issue--in the opposite way you might think. Older towns in the mid-west are spaced apart roughly 6-miles because you would only be able to travel by horse about 20-miles in a day. Naturally, you would want to be able to do "things" in the next town so you don't travel the entire day just to turn around and go back home. So, towns spread out about 6-miles and slowly grow. Bigger towns grew faster and smaller towns stayed the way they are. Bring in interstates and railroads and suddenly some towns, if they had nothing "going for them" never grow again or shrink--they're completely bypassed.
People leave town for the opportunities, so opportunities have little need for these towns except for the occasional externality: e.g. a massive chicken processing or pet-food rendering plant no one wants to live near.
Dollar General has found a dirt-easy strategy and picks up this easy money because this is essentially a structural problem with people who have tied themselves too closely to the land and governments that can't solve their problems.