https://wccftech.com/fallout-76-steam-issues/

Fallout 76 will be the first entry in the series to not get a release on Steam, as it will be available exclusively on Bethesda.net. In a fresh interview, Pete Hines revealed the reasons behind this choice.

According to Hines, with Fallout 76 being an always-on game and a service, Bethesda felt that they needed to have a better one-to-one relationship with customers without having to go through another third party.