Have you had the McRib? It's in this limbo state of being popular enough to warrant introduction into the McDonald's menu periodically, but not popular enough to be a permanent staple.
Basically people will especially go out and get the McRib when it "comes back" and then get tired of it.
If McDonalds was limited to only one location, there would almost certainly be long lines, waitlists, and reservations. The ubiquity of McDonalds, and the fact that they can meet demand enough so that no one needs to wait more than a few minutes contributes to the luxury of not having to wait for a Big Mac.