RHOA has been on nearly 3 times as long as Potomac. They like to keep the main girls long after they should be put out to pasture.
They also barely bring in new girls, and production makes them compete for peaches without revealing who made the cut until the end. Is it the Hunger Games?
Notice the other cities usually just throw someone in the mix to sink or swim, and they announce they're apart of the cast long before filming ends. Other cities usually have more authentic relationships between cast members. Potomac works largely due to that. Monique, Candiace, and Ashley were outsiders, but Gizelle, Robyn, and Karen (and Charisse) have all known each other for 10+ years outside of the show.
I think a core group has to know each other pretty well, and then you sprinkle in a couple of outsiders over the course of a few seasons to mix things up. Either way, like any show after 10+ seasons, it will get stale.