They have very different approaches to designing worlds. Fallout 3 and 4 have much more tangential and almost self contained wackiness going on that can be tonally all over the place, and FNV tends to be built all in support of a single tone and while it has wacky elements a bit more grounded.

Different approaches. What this means for me is that while I find the Fallout 4 main quest incredibly poorly written, I can largely ignore it because there are so many unrelated tangent stories that I do like a lot, and locations to explore in a kind of themepark of a game world. Whereas FNV doesn’t quite have that themepark level of variety as it is building a world that’s trying to be immersive in support of the main story.