A key difference between private servers long ago and private servers today was the shear quantity of them. When I first started playing on private servers (Wrath was still early into its expansion cycle) there were not many options for a blizzlike server so the community was focused on only a few realms. Nowadays, it is so easy to release a decently put together server that just about anyone can (and several indeed do) do it. We end up with too many servers and very few able to maintain a lasting community. This is not the only difference, the game as a whole is less popular now than it was during its prime and every new expansion can split the numbers further. Still, if it were possible to limit the number of servers to match the actual demand for each expansion, there would be plenty of players to create strong communities.
Obviously we can’t control who opens a server and rarely are we organized enough as a community to effectively boycott realms (elysium’s Vanilla drama and sunwell’s TBC stand out as possible exceptions) so there’s sadly not much we can do about it. Also, I know this isn’t a radical new idea but reading through the AMA yesterday this point stood out to me as a variable that is very easy for blizzard to control but nearly impossible to regulate in the private scene.