IIRC the guy who was doing the official Linux builds of the old id games was also the one who was publishing the code, so that's probably why.

They could never publish everything they had internally, as they used proprietary or patented bits in places, so the code always needed cleaning up before GPLing it. That's why Doom engine source was released without the sound, the Doom 3 engine had the shadowing code replaced, Quake 3 code didn't have the single-player parts because people were still licencing it in the mid-00's, and other such stuff.

And yes, Doom was originally developed for *nix, I think they were using NeXT machines or something like that, for development.