From what I read, SteamOS isn't really intended to run on any device other than official Steam devices. In particular I read that it doesn't support nvidia GPUs at all, since all official Steam devices are AMD.

Also, I have a pretty unusual setup: my machines netboot from an shared iSCSI volume, setting up a local copy-on-write overlay on each machine.

SteamOS is based on Arch, so I'm sure it would be possible to make it do anything Arch can do. But I don't know Arch -- I know Debian. So I was a lot more comfortable installing Debian and tweaking it the way I needed, then installing Steam on top.