Mono=one poly=seller... and last I checked Steam is not the only seller of video games. They aren't even the only seller of digital video games. They aren't even the only seller of digital video games for SteamOS.
They are the largest because they do what's right by their customers and employees. As a 'for instance', I bought Portal 2 for the PS3 many years ago. I no longer have my PS3 but I can still play Portal 2 (as well as Portal which was just thrown in for me) on any PC.