Modding is a part of it. The modding scene is a massive part of Elder Scrolls and Fallout. With Fallout 4, people had limited mods ready on day one. People have been able to transfer some types of mods between Skyrim and Fallout 4 relatively easily.
The other part is probably time. Getting everyone trained up again and then transferring all the systems and assets over would probably add a huge amount of time to development, where we already have to wait many years as it stands.
And, to be honest, Fallout 4 right now is incredibly stable and by far the best shape any of the games have ever been. They've obviously done a lot of work. The major problem is how inefficient the engine is with certain modern techniques, such as the way it handles shadows. However, it could be that they think that will be solved with next gen hardware upgrades.
As for the usual "I hate Bethesda, they're going to crash and burn" people, keep telling yourselves that. The next Elder Scrolls game is going to sell like hot cakes.