https://www.gamespot.com/articles/heres-why-fallout-76-isnt-coming-to-steam/1100-6462400/

One of 2018's biggest games, Fallout 76, won't be available on the world's largest PC platform, Steam. Instead, Bethesda is releasing the game on PC exclusively through its own Bethesda.net launcher. The move makes business sense, as Bethesda won't have to pay Steam operator Valve its cut of sales, but the announcement shocked some fans as previous Fallout games like Fallout 4 and Fallout 3 launched on Steam.

So why won't Fallout 76 be available on Steam? Bethesda marketing boss Pete Hines told GameSpot that the company wants to have a "direct relationship" with Fallout 76 players due to the game's always-online, multiplayer focus.