A post on reddit by Timothee Besset reflects on his years at id Software, where he worked on programming projects and Linux ports until his departure a year ago (thanks Ant via LinuxGames). In discussing his departure he says id lost interest in the PC, and him leaving is not the reason for there not being a native Linux build of RAGE:

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 :)