Quake has a software renderer, that probably helps.
But most importantly, ID software released the original source code for it quite early. Not that many game development companies do that. I would've loved to see more Blood ports, but I don't think the source was ever released? So I believe the existing ports were reverse engineered. Duke Nukem 3D source was only released in 2003 it looks like.
The quality of the source code of Quake 2 is quite high. Doom is.. a bit messy but OK. There are already abstractions that make porting easier. I'm guessing Quake (haven't looked at the source yet) is somewhere in between. That helps too.
But most importantly, ID software released the original source code for it quite early. Not that many game development companies do that. I would've loved to see more Blood ports, but I don't think the source was ever released? So I believe the existing ports were reverse engineered. Duke Nukem 3D source was only released in 2003 it looks like.
The quality of the source code of Quake 2 is quite high. Doom is.. a bit messy but OK. There are already abstractions that make porting easier. I'm guessing Quake (haven't looked at the source yet) is somewhere in between. That helps too.