If you'd been putting off buying Aliens: Colonial Marines or the 2010 version of Aliens vs. Predator on Steam, then you may have missed your window of opportunity, for the time being at least, as both games have been delisted on Valve's digital storefront. It's probably just something down to licenses, but with no official word from Sega on why the games have been removed, we're can't be sure. At least you can still buy Alien: Isolation, which we'd thoroughly recommend you do so if you haven't already. We'll update you when we know more.
Update: Worry not, dear readers, for Aliens: Colonial Marines and Aliens vs. Predator have both returned to Steam. You may now resume your daily business.