Basically they have to modify the maps to implement HDR; for something like CS:S that isn't to hard to do over time, but for Half-Life 2, there's a crapton of maps and IIRC, it'd be like a 1 gig download. They want to add it, but just not right now.

Q: With Day of Defeat: Source we saw for the first time the much anticipated HDR and most people fell in love at first sight with it. We also know that all future Valve products will include this technology. Are there plans to add HDR in already released products such as Counter-Strike: Source and Half-Life 2 itself?

A: Counter-Strike: Source, definitely. We'll be rolling out HDR versions of the maps over time starting very soon. As for Half-Life 2, not in the short term, but possible at some point.

Q: The last Steam weekly news update stated once again that The Lost Coast will be released soon. Still, many would like to know more about the Counter-Strike: Source and Day of Defeat: Source content that was mentioned in the same announcement.

A: While we haven't done a significant release for Counter-Strike: Source in a little while, we've been continuously building content for it. There are two new maps on the horizon for Counter-Strike: Source, de_nuke, and cs_militia, which will bring the total number of maps we've released to 18. The team has also been busy working on getting the rest of the player models built, which we'll also be releasing soon. November should be a fun month for Counter-Strike: Source fans. On the Day of Defeat: Source front, we have a few maps currently in development. There will be a mix of new maps and a Source adaptation of an original Day of Defeat map coming before the calendar year is over.