It looks as though the 2010 release of Aliens vs Predator and Aliens: Colonial Marines have both been pulled from the Steam marketplace.
This could be due to a licensing issue, and both may be put back on the marketplace by SEGA in the future; however, neither are available on Green Man Gaming either per a search we conducted.
Aliens versus Predator Classic 2000 is still available through both services and is also still available through GoG.
Via Kotaku UK.
Read more here: VG247