the game is dying because its player retention is horrible. its player retention is horrible because vanilla blockland is hard to get into. vanilla blockland is hard to get into because it's a bad game. vanilla blockland is a bad game because it's extremely barebones and requires mods to actually have a decent experience playing it.
there's also a learning curve for creating anything that makes the game fun, so people who can't or don't want to learn the stuff required to make the game fun rely on other people to make content for it, and that only goes so far. blockland is essentially just a chatroom with some building mechanics, so it's only really fun as long as there are enough people playing it.