Device made with software specifically for purpose performs better than generic machine with generic software designed to do a wide range of things. All of my machines are on Linux distros, but this just seems like a no brainer to me. It's like years ago when the mustang had a 4.6L V8. It was the same engine used in the Ford explorer. Will the Mustang beat the Explorer to 60, of course. But the Explorer will also transport 5 people to the beach with coolers and beach gear and drive in the sand.
It's good that SteamOS is doing well, but the variety of tasks people are using Windows for cannot be performed on SteamOS.