1. You have a handful of nice small towns each with their own little commercial center with a small grocery store and a collection of other shops and restaurants.
2. Wal-Mart wants to move into the area. Their efficiency is driven by logistics and scale. They don't want to open a small Wal-Mart in each town that they have to drive trucks to. So they pick one of those towns and put a mega-Wal-Mart there.
3. People in the neighboring towns start going to that Wal-Mart occasionally because it has a few things the smaller local grocery store doesn't have.
4. Business dries up at the local grocery stores. They already operate on razor-thin margins so it doesn't take much to put them in the red. They close.
5. Now even the people who don't want to go to Wal-Mart have no choice.
6. People tend to visit the other shops and restaurants as an additional errand tacked on to their necessary grocery run. Without a local grocery store anchoring those trips, people no longer stop at the restaurants or other shops. Those satellite businesses close down.
Ta-da, now you've got a bunch of dead towns with no functioning city center and one bigger town with a Wal-Mart. Traffic gets worse for everyone. Wal-Mart rakes in the cash because instead of them having to pay for last mile product shipping, customers do it for them by driving thirty minutes from their food desert dying town to the nearest Wal-Mart.
Did I mention that all the people who used to work at these small stores and restaurants now have shitty jobs at Wal-Mart and awful commutes?