The Unreal engine is one of the most prolific pieces of technology used in the gaming industry, and the fact that the developers from Epic have just added official support for SteamOS and Linux is actually one of the biggest leaps for the open source platform since the release of Steam.The Linux desktop has a really small share of the market, about 1.5% if we take into account a moderate estimate, so it's understandable that publishers and game developers don't really consider Linux as a valid platform.
Valve was the first company to break the ice in the gaming industry for the Linux platform and hundreds of quality games were released in just a year and a half, which is actually more than in all the previous years combined.
There is one problem though. No major publisher or game developer has released triple-A games. There are a few that push the Linux platform to its maximum potential, like Metro: Last Light, but it needs a company with a lot of resources and muscle. Electronic Arts and Ubisoft would be perfect candidates.
Another problem is that it's not easy to make games for the Linux platform. A number of smaller engines already provide support but, until now, none of the heavy hitters expressed any interest, with the exception of Crytek. CryENGINE is a beautiful engine and games that use it will make it to Linux someday, but the engine is not widespread.
The Unreal Engine on the other hand is quite prolific and you can find it in hundreds of games, especially for the old generation of consoles. It's a cheap solution and it looks amazing, which means that it is the favorite tool of many big studios.
Now, Epic has just published the Unreal Engine 4.1 Release Notes, which clearly say that developers can now package their games and deploy them to SteamOS and Linux without any major issues. Developers also explained that, for now, deploying projects to Linux requires compiling source from GitHub using a Windows computer, but that is about to change.
Having the Unreal Engine on the Linux platform would mean a lot for Linux as a gaming platform, but it also has greater implications. One of the problems Linux faces right now is the lack of big titles and development parity with Windows and Mac, but if the Unreal Engine breaks the dam and the games start pouring through for the open source platform, we will see a shift in the market share for Windows and Linux in just a few years.