- iOS/Android are huge. The Microsoft monopoly on client devices has been broken. Years ago, most game developers were fine to use Microsoft exclusive tools that produced apps that were Windows exclusive. Recently, that has really changed, and there is a much stronger push towards platform neutral APIs. For things like 3D graphics, sounds, and keyboard/mouse input, there really is no reason to use a non-portable single platform API over a platform neutral one.
- Linux in general, is in way better shape than it was years ago. Today users get can get the latest and greatest web browsers like Google Chrome, rock solid Flash/YouTube, and "it-just-works" installation and hardware compatibility (mostly). None of these things were true in the past. Years ago, Linux wasn't practical compared to Windows for many usages. Today, beyond cost and ideology, it is just a better product for many productivity uses. Anecdotally, I see many programmers and tech types are switching and are surprised by how much better it is. The command line is *way* better, the software repo is awesome and makes it easier to install zillions of tools like python and git and keep everything patched and avoid malware/adware. For command line centric programming tools like python, ruby, octave, java, scala, git, etc, Linux just makes more sense. I would even say that things like Microsoft Word that locked a lot of people into Windows have been somewhat replaced by markup tools like markdown and TeX.
Now that people are using Linux in larger numbers as their main work/school productivity machine, games are sure to follow. Linux doesn't need a Steam at all for games.