From his Reddit post:

http://www.reddit.com/r/linux_gaming/comments/17v6b6/rlinux_gaming_this_is_why_i_discourage_the_use_of/c895wvs


"I wasn't able to allocate time to work on native Linux or OSX builds for idTech5 stuff in the last few years I spent there (basically after idTech4 Doom3/Quake4/ETQW). There were plenty of reasons that led to me leaving, but me staying wouldn't have changed much in that regard.

They simply lost interest towards the PC in general during the idTech5 cycle, and they don't have anyone there with Linux chops anymore. But Valve seems to be doing great at bringing AAA Linux stuff to the market now, and there's plenty of indies doing high quality stuff. So here's to the new generation "

Simply lost interest in the PC. Explains a lot eh'.