Over the years, several notable publishers have begun moving away from Steam in an effort to grow their own platforms. Bethesda has had its own launcher for a while but hasn't done much to really push it. That is changing this year, as Fallout 76 will be exclusive to the Bethesda.net platform.
Bethesda.net has been around for a couple of years and received its first big push with Fallout Shelter, which eventually landed on Steam after a year or so of exclusivity. At launch, Fallout 76 (and its beta) will be exclusive to Bethesda.net, so you won't be able to pick it up on Steam.
This news was revealed via the Fallout 76 beta FAQ, which states that “the beta and the full game will be available on Xbox One, PlayStation 4 and on PC (via Bethesda.net only)”. This is a surprising move, as most of Bethesda's big games do launch on Steam, and sell very well over there.
There is the chance that Fallout 76 does make its way to Steam eventually, particularly if a sales hit occurs. For now though, it looks like those planning to buy will need to do so direct from Bethesda.
KitGuru Says: Ubisoft, EA, Epic Games and Blizzard have all had success building their own platforms on PC, successfully avoiding Steam royalty fees. Now, it looks like Bethesda might be joining that growing list. Fallout 76 seems to be a test run for this strategy, so if all goes well, we'll likely see this practise continue.