Porting games to different platforms takes time and money. It's very possible internally at Valve there are a few interested in a long term vision of seeing the engine, games, and Steam itself run on Linux. But all this takes time and money. However even if they want it to happen it doesn't mean it can happen over night. Often when porting to other platforms you make tiny progress here and there by replacing one bit of windows code with another bit of cross platform code. Or one windows library here with another cross platform library (i.e. webkit). Eventually the light at the end of the tunnel gets brighter and you realize it's actually going to be possible. Who knows where Valve is in that tunnel. My guess is that with the Mac port done it must be technically within grasp and largely coming down to marketing and financial decision planning for if and when it makes sense to release.