I never said they need Linux to succeed. They want Linux to succeed or else they would not push their SteamOS and they made public in the past that they did want Linux to succeed and therefore transition from Windows to Linux.I think Steam is doing quite well on Windows and even macOS to a degree. I doubt they really need Linux to continue to succeed.
With that said, since they do have a desire to play in the Linux community, Ubuntu is the logical choice to work with in this area. Ubuntu is the the most well-known, ubiquitous, distro to people who know very little about Linux. Plus, it's one of the top easiest distro's to use for newbies.
Sure there is a voice from the Linux gaming community to have Steam on the platform and I think that's good. But I won't fool myself into thinking Linux will save Steam or vice versa.
Just a small sample: "Linux has never been a hot gaming platform. Valve, creator of the Steam game engine and network, has long wanted Linux to replace Windows as the PC gaming platform. Indeed, Gabe Newell, Valve's CEO, once said, "Linux is the future of gaming." That didn't happen, but Valve didn't give up on Linux. Now, Valve is doubling down on Linux, by proposing changes to the Linux kernel to make it more gaming-friendly."
https://www.zdnet.com/article/valve-proposes-game-friendly-changes-to-the-linux-kernel/