Not available on Steam for now.
A little over a year after it first launched, Bethesda’s excellent Fallout Shelter, the mobile game spin off that places the player in charge of managing an underground Vault-Tec vault, is now available on PCs too. The version of the game available on PC is the same one as the iOS and Android versions, which recently got a major update that added substantial new gameplay mechanics, such as quests, to the game.
The game is not available on Steam, however- instead, players must download and install the Bethesda.net launches, since the game is apparently not available on Steam. It is unknown whether the current Bethesda.net exclusive launch is a temporary thing, with the game coming to Steam eventually, or if Bethesda plan to keep this one exclusive to their service.
Fallout Shelter is a pretty good, although a little shallow, game, but it is one that fans of the Fallout universe will doubtless enjoy. If you count yourself in that number, then by all means, go ahead and install the game, and enjoy.
— Fallout (@Fallout) July 13, 2016