https://www.gamespot.com/articles/why-fallout-76-changes-the-series-lore-including-t/1100-6462382/

Fallout 76 takes place before all of the other games in the franchise, featuring one of the control Vaults that was meant to re-colonize the surface after the radiation threat had passed. That puts it in a unique place that could run into continuity errors with the existing canon. Bethesda marketing boss Pete Hines explained to GameSpot how the studio is balancing its storytelling needs with the story so far.

"Our developers take things like lore and canon seriously and if they're going to do something they're going to make sure that there's a real and defensible reason for it," he said. "We have proven with Elder Scrolls games, we're willing to say 'Well lots of people will say things happened one way,' and the opposite or something else could entirely be true. So there's no question that we've gone back to change things to fit what developers have wanted to do and not be beholden to something that somebody wrote 20 years ago even in franchises that we created like the Elder Scrolls.