Two Sega-published xenomorph-based games - Aliens: Colonial Marines and Rebellion's 2010 version of Aliens vs Predator - are no longer available to purchase directly through steam.
Some might think it's because they're pap, but it's likely because of licenses expiring and all of that silly gubbins that's hard for us normies to understand.
Basically it's a regular thing for licensing issues to stop games from being on sale via certain outlets - we've seen it with a bunch of Activision's licensed titles in recent years, as well as a similar situation with Crysis 2 the other year.
It's alright though, you can still buy both titles in physical form, and digital stores like Green Man Gaming offer both that can still be activated on Steam. (Here and here, in case you want to punish yourself).
And it's even more alright because you can continue to ignore a terrible game (Colonial Marines) and a devastatingly average one (AvP), instead picking up our game of the year, Alien: Isolation, or the infinitely better non-2010 version of Aliens vs Predator.