Because they could sell them as new games.

Fallout is hot right now thanks to the show, and from a business perspective its kind of crazy that they didn’t plan to have something available to capitalize on that interest. A Fallout 1 & 2 remake or remaster would have been an easier option than a whole new game. And it’s the kind of thing you can outsource to another studio, so it doesn’t have to disrupt their current plans.

If I were some soulless executive at Microsoft, I’d have been getting this put together the second I saw that the show was a huge hit. I’d be trying greenlight remasters of basically all the games, plus a new non-numbered game in the series that could have limited scope but keep the same basic flavor, and maybe a new game in a different genre altogether. Things that could be handled by other developers and pushed out over a reasonable time frame so we could at least have something to announce before season 2, while letting Bethesda keep Fallout 5 for whenever they finally get around to it.