XCOM: Enemy Unknown Elite Edition arrived on Mac on April 25, one place it didn't launch was Steam. A representative of the Mac version's developer, Feral Interactive, told Joystiq that it "does not release games on Steam."
This isn't the only game that Feral has shipped on platforms not owned by Valve, as the developer is credited with creating Mac versions of BioShock 2, Grid and both Batman: Arkham City and Arkham Asylum, among many other games. When asked about whether any games from its catalog would launch on Steam at any point, the representative said they "do not talk about future products."
Feral is offering XCOM: Enemy Unknown on the Mac App Store, among other online retailers listed on its site.