>these places are thriving in places less dense and more impoverished than Pittsburgh

The traditional food stores (supermarkets, chain convenience stores) tend to avoid impoverished areas, so the local bodegas there face less competition. And lower income folks are less likely to be able to travel to a full service grocery store. (no car, poor public transit). The lower density areas also tend to have less public transit options.

Maybe these are some of the reasons they are thriving in the cities you mentioned? In addition to the reasons I mentioned previously?