Valve Software is doing their big things with Linux now and other game companies are looking at Linux for gaming, but id Software is doing the opposite. Previous versions of the id Tech engine had Linux support and their games up through Enemy Territory: Quake Wars had native Linux builds, but then with id Tech 5 and their premiere Rage title there wasn't any Linux client. id Software's "Linux guy" also left the company.
Now during Carmack's annual keynote at QuakeCon, he didn't have anything good to say about Linux -- the open-source operating system hasn't produced positive results for them. "Linux development is another story altogether. Even though Valve is now actively pursuing the Linux market, iD has been there before, and just has not seen positive results. Remember how many past titles from iD actually ran on Linux, and for how long these were supported? John says that Linux development simply does not pay the bills. It creates goodwill among the Linux crowd, but that is about it."
Sadly then beyond Rage not coming to Linux, Doom 4 -- another upcoming id Tech 5 title -- probably won't make it to Linux either. One can only hope that id Software will eventually change their mind. Otherwise, when id Tech 5 is hopefully open-sourced many years down the road, at that point independent Linux developers might put together the Linux engine support.
More details on Carmack's comments are available from this forum thread. Embedded below is the QuakeCon 2012 keynote in full.