Because Quake 4 was limited to PC and a mediocre port on X360 and Doom 3 is limited to PC and land locked XBox 1 copies.
Even if its only for the single player mode and slight enhancements, additions to the story it should be done so that console crowd can appreciate the game instead of showing up and being forgotten.
A remake imho can do wonders for a franchise that has little presence in a console generation.
It's an id game. Even if Doom 3 wasn't received that great once all was said and done I think most people are at the very least curious and would check reviews and demos of their games. So I see very little point in making ports of old games, games which (IMO) were not very good in the first place.