If I'm not mistaken, Arch Linux only supports x64 (excluding the unofficial ARM port), so I wonder why they've chosen to go with it rather than Fedora or Debian.

My guess: Fedora is in the tight grasp of RedHat, and Debian perpetually ships old packages.

Steam/Valve are doing lots of work on various parts of the stack upstream, that they'll want to get hold of as quick as possible and not be limited by outdated dependencies.