It sounds like you have some holes where you used to have inactive cities. Inactive players who have not purchased Diamonds nor reached the third tier, are removed if they've been inactive for 30 days.
That makes room for new players. The US worlds are still pretty new, so we're still seeing a lot of bump and run visits, but if previous experience is still relevant, it's been taking most of 2 weeks for the empty cities to be reoccupied.