What happens if the developers don't like the mod?
Software that is well-maintained should make its decisions based upon the consensus of the users.
Also then how would players pick and choose which mods to use?
If having such a choice would benefit the game (determined by the consensus of the users), it could be implemented as an optional thing.
a lot of mods would very much not fit in the main game
Then in that case, they fill a niche distinct from the original game, and would be perfect for a fork.