Counter-Strike: Global Offensive is one of Valve's more recent titles that was officially released last August as their fourth CS title. Counter-Strike: GO improves some of the assets found within Counter-Strike: Source and also offers a wealth of new content, engine improvements, and other enhancements.
I have heard that Counter-Strike: Global Offensive is currently being tested on Linux and that the port is shaping up fairly well, similar to their already debuted Linux games. As some independent confirmation of that aside from what I've heard, a few days ago there was an Intel Mesa driver commit pertaining to an optimization that also benefits "CS:GO." The Mesa commit is a GLSL optimization to flatten simple nested if-blocks, where for CS:GO they can save a few instructions from being generated by this flattening process. This was the first Intel Mesa commit pertaining to this Valve game.
It's not a surprise that Intel's Open-Source Technology Center developers are already working on optimizing Counter-Stike: Global Offensive, just as the in-house developers at AMD and NVIDIA are also doing to their proprietary drivers.
Left 4 Dead 2 still hasn't debuted on Linux while Intel OTC developers have been running it and making L4D2 driver enhancements going back to last summer. Intel developers even have had access to Valve's Perforce repositories with the game source-codes.
For CS:GO on Linux, things hopefully won't take as long as it has for L4D2 and some other Source Engine titles. With this being one of Valve's newer titles, they're likely trying to get it out the door at a more pressing rate while the game is still fresh amongst gamers.