Basically the issue is that every six months they break all mods. Many projects over the years got abandoned after an update, or they were just never able to make progress because every six months they’d have to spend weeks patching.

There are some big mods still, but they’re mostly just content additions. Anything that does overhauls, or has lots of overlapping systems, is doomed to failure unless they want to target a specific version of the game and never update. There was a big story mod awhile ago that decided to lock the game version that they supported, but it died when some dependencies updated to the new game version.

Modders work in their free time, so they can only make real progress when they have a stable base for a long long time.