Inactive neighbor purges hurt. I experienced the notorious Arendyll purges last summer, when many players sat for a couple of months before new neighbors began to arrive. <sigh> Last week, many neighbors disappeared from my Aren neighborhood again.
This time, I'm in a strong fellowship, so the impact is lessened, however ... I still have a handful of active neighbors toughing things out on Aren. Our cities are located out on the fringes of the Arendyll server. It makes a big difference where your city is placed on the server for each world. Winyandor and Elcysandir are easier to play and a LOT more fun, I might add - because the first player on both of those servers, is one of my neighbors - meaning I'm close to the center of that server's hexagon spiral. It matters, because a lot of players who tend to be more active, join a server/ world right away when it opens.
There's a fair number of players that have multiple accounts on each server/ world. I won't speculate on all the reasons, but know one is - players don't like the boosted goods they receive when they log onto a server/ world, so they try again. Most players leave the cities they don't want, sitting inactive. Some players are playing those multiple active accounts.