OK let's add some extra info to why these titles weren't seen as that popular on Linux though:
Quake I - Released in 1996, and according to wikipedia Quake 1 had a linux port released in 1996, however the retail Linux version 'The Offering' didn't come out until 1999. In three years I'm sure people that wanted to play on Linux were already doing so using the existing port. id software had no real way of tracking Linux sales of the game.
Quake II - Released in 1997. Once again, the linux port for this game existed before it was officially sold in a retail box for Linux, which wasn't released until 1999. id software once again had no way of tracking real Linux sales.
Quake III - Released in Dec 1999 (really!? that long ago?!). I believe Loki Games were doing an official port to Linux and also selling a tinned box copy. Unfortunately they shipped it a little after the id software retail release of Quake III. Once again, most people waiting for the game would have just bought the Windows release first, and then downloaded the Linux binaries for it later.
Seriously, how did id software gather their stats on Linux use of their games considering the above? FTP site downloads of the binaries?? They were mirrored in heaps of places!