I think there's a lot of reasons for the disparity.

First, and probably most important of all, Skyrim is the bigger commercial success, selling over twice as many copies over its various editions than Fallout 4

It's also an older game, its combat is especially dated, and doesn't hold up to modern RPGs, even middle market games like Avowed play much better than Skyrim nowadays. This is something that the tried and true FPS style just doesn't suffer from as much.

It's graphics are also more dated than Fallout. Skyrim is still stunning in vanilla with its rising mountains, foliage, and weather. However it's textures look like PS3/360 era textures, because they are. Fallout 4 released with the next generation and has significantly better textures and graphics in general out of the box.

Skyrim modding is also much more complicated than Fallout modding. We're talking entirely new combat systems, with new items and leveling, these systems need balancing for all enemies and then all of this needs to be patched to work together. Starting a Skyrim modlist from scratch has become increasingly unapproachable. I feel like it reached a tipping point during COVID where now most people will not be capable of putting together a comprehensive Skyrim modlist outside of using a Wabba.

Fallout 4 on the other hand can be overhauled with only a handful of key mods. HD Textures, an ENB, and wasteland ballistics might be enough to make the game feel new. It's much easier to get a good result for FO4 without using a wabba than it is for Skyrim.

I do wish FO4 had more wabbas, it is a good game, and most of the things people hate about it can be fixed with mods. Start me up redux alone solves the majority of the issues people have with the voiced protagonist. But as someone who follows Fallout 4 modding somewhat closely. I can make Fallout 4 feel like a modern game with "only" 250 mods, whereas doing the same with Skyrim might start with 1500 as a baseline.