It's -- you guessed it -- a legal issue. It all started with a news post on Good Old Games, which announced the removal of Fallout 1, 2, and Tactics from the site's catalogue. Shortly after, the trio of post-apocalyptic RPGs was "removed" (the actual pages stayed, with only the buy links being deactivated) from Steam as well.
While previous buyers can still access the games, there is currently no way to buy new copies. To find out why this happened, we need to revisit the "Great Fallout Legal Battle".
It happened a few years back, between Bethesda Softworks, developers of the Elder Scrolls series and current Fallout rightsholders, and Interplay Entertainment, publishers of all Fallout games released before 2008. At the conclusion of this legal battle, Bethesda was awarded full control of the Fallout franchise, but Interplay was allowed to continue selling the first three Fallout games until December 31, 2013 -- four days ago.
So, when can we purchase the first three Fallouts again? Soon, according to Bethesda Softworks, who recently announced on Twitter that they're already working on bringing the games back to Steam -- though when asked about GOG, the company maintained that their "current priority is to return the games to Steam," neither confirming nor denying whether the games would be back on GOG. Hopefully, they'll get right on that as soon as possible.