SteamOS is an OS for gaming consoles. It’s specifically tailored for gaming and it has controller-friendly UI.
You can game on regular distros, but you need to install and open Steam, download games, and, then, launch them, before you can grab the controller.
It’s a little more than that.
SteamOS also uses an immutable filesystem and the system updates as a whole. Because of that, there is no risk of something updating separately and breaking compatibility.
It’s fairly common for things to update on regular linux distros and break e.g. anticheat support in Proton or some other thing.
Another thing SteamOS does, at least on the Steam Desk, is actually using two partitions. The updates are always installed to the inactive one, so there’s always one image that’s known to work. Even if an update fails, the device will simply boot into the intact OS image. Regular distros usually don’t have much in terms of fail-safes, so if things break, they have to be fixed manually.
Basically, SteamOS is trying to be as reliable and “hands-off” of an OS as possible to provide best console-like experience.
I think it’s really more about the extensive Proton compatibility testing.
