They probably have other things in the pipe, such as the never ending Team fortress 2 and a possible Half-life 2. Sure a Half-life Linux would sell awesome RIGHT NOW, but woulden't that need a shitload of recoding for Linux? Valve may be thinking that Half-life is running out of time, and by the time they get a Linux version released, it will be on it's way to death.
Just because they won't release for Linux does not mean they hate Linux. Heck, they probably use Linux around the Valve offices for some things. Chances are some developers are wary of jumping on the bandwagon... because there isn't enough developers jumping on right now.