This year's addition to Sega's series of Alien-related video games, Alien: Isolation, was easily the most successful in recent memory. Playing as Ellen Ripley's daughter, Amanda, players were left to sneak and scurry through the space station Sevastopol in constant fear of a deadly encounter with the alien stalking them. The game's atmosphere and retro homage to the aesthetic of Ridley Scott's original film made Alien: Isolation a success in the eyes of most critics.
The same cannot be said for Aliens: Colonial Marines, the first-person shooter developed by Gearbox Software which drew plenty of ire - and, eventually, a class-action lawsuit - upon its release, as it became clear just how misleading the footage shown at conventions and trade shows had been. Sega eventually paid a $1.25 million settlement, but since Gearbox has denied responsibility for consumers' disappointment the lawsuit is still ongoing.
For reasons that may or may not be connected to this messy legal saga, Aliens: Colonial Marines has now been removed from the Steam store along with 2010 title Aliens vs. Predator, which was developed by Rebellion. GameSpot speculates that this may be due to the expiration of certain license agreements between Sega and Twentieth Century Fox, but Alien: Isolation remains in the store.
The games are still available to play for anyone who already bought them, but they are no longer available to purchase from the Steam marketplace. This isn't exactly a great tragedy, since Aliens vs. Predator received decidedly average reviews as well (though without the same kind of heat that Aliens: Colonial Marines got). At worst, some unsuspecting innocent has just been saved from buying a terrible Aliens game during the next Steam sale.
Games being delisted on Steam is a fairly common practice (here's a full list of the titles that have disappeared) and it's possible that the two Aliens games could come back again at some point in the future, as The Walking Dead: Survival Instinct did after being temporarily removed.
Sega has yet to comment on the removal of the games, and for now it seems like licensing issues are the most likely explanation. Let's just hope that Alien: Isolation doesn't end up going the same way.
[Update: As of the evening of January 7, 2015, both Aliens: Colonial Marines and Aliens vs. Predator have returned to Steam.