Debian has the Debian Free Software Guidelines and the community as whole is less supportive of proprietary software, not to mention that it's all volunteers. I

Debian has the Debian Free Software Guidelines and the community as whole is less supportive of proprietary software, not to mention that it's all volunteers. I think Valve would rather work with a company-supported distro like Ubuntu, which focuses on just working. In any case, the client may very well run on Debian, it just won't be officially tested or certified by Valve (for now).