I would assume, without knowing much about game development, that making a game from an existing IP is less time consuming than having to start your own. Expecially with a game like Fallout.

There is alot of things that they can duplicate from the previous Fallout games and tweak them a bit. The chr creation, stats, lore and story.

Having to make that from scratch I would assume is more work.

I do think though that Bethesda's developers, or the leading ones atleast, liked the first Fallout games. And saw a way to continue a great series in their own way.