I know what you're thinking, Bethesda doesn't make multiplayer games. Well, technically, you're right with the exception of the Elder Scrolls Online. On the other hand, they do discuss multiplayer for every one of their games prior to release. There are many reasons for multiplayer in games. With the advancement in technology gamers want to play as many games online with their friends as possible. It's also a money maker and a way to expand the gaming experience for consumers once the single player campaign is completed.
In a recent interview at E3, Director Todd Howard revealed, "Believe it or not, every time we do a game we design a multiplayer mode just to see what we would do. Then we go, 'Yup, we shouldn't do that. That's an awesome idea that is way too distracting from the main thing we want to do, which is single player. I could see ways it would work, but development-wise, they're very different experiences."
Apparently, most of the work is done in pre production before any of the serious programming begins. It's all done old school, pencil and paper style to see what they could do. Once it's all worked out they decide whether or not it can fit into the game. Fortunately, it was scrapped for Fallout 4 prior ot final production. As much fun as multiplayer is, Fallout is one of those games that is iconic and may very well see a detriment with the addition of it.