It's simple, Fallout 4 was marketed and developed as a mainstream title. Several things about the old Fallouts were potential turn offs for the average consumer. They were gritty in a way that you either find great or distasteful, Fallout 3 had an extremely long development time which left people with high expectations, only to be let down, and the Fallout name just wasn't household like it is now. At the time of release, Fallout fans were very excited for the game, and many people wanted to join in on this excitement even if they hadn't played the series before. Who doesn't love a hype train? People saw it as essentially an open world, crafting based shooter with cold-war-gone-hot vibes, decent story elements, and a cute dog to wrap it all up. Tell me that isn't a cocktail of mainstream appeal. It was a no brainier and a marketing hit. Bethesda knew exactly what they were doing and they hit the nail on the head. Unfortunately nowadays the big money lies in games like Fallout 4, and not the older interactions.