It’s also worth noting that Petersen’s MoGas STC isn’t just a “paper STC” depending on aircraft model. For the PA-28s, it requires replacing the standard fuel pumps. And then there’s the requirement that the fuel not contain any alcohol, which is hard to find in my area. Their page also says that mogas can be more volatile (reid vapor pressure) than avgas, which can lead to vapor lock and/or a higher probability of carb icing.
The point being, the mogas STC may be quite valid for some, but for others it may make less sense. This just means that it’s not a replacement for 100-octane avgas, with a somewhat limited market, so it’s no wonder very few airports sell compatible mogas.