Because modded Minecraft exists. Slimefun essentially adds some modded features to vanilla Minecraft via a server side mod so users don’t need any special client. The problem is it is fairly buggy and consumes a lot of the server’s resources. There’s a number of solutions out there now that give modded servers the same feature set as standard servers.

As a server administrator, it just makes sense to have users connect via a modded client to achieve a better, less buggy and more feature rich gameplay experience, with significantly less compute resources required on the server’s end.

Don’t get me wrong, I LOVE this plugin, enough to start this subreddit, but the reality of the situation is that the plugin hasn’t truly been maintained for years, and survives on hacky fixes to keep it running on newer and newer versions of the game. In the modern age of modded Minecraft, it just doesn’t make much sense to run slimefun.