Tremulous as a project was never about engine development, and certainly not about creating high detail graphics. To compare it to XreaL, or Overdose is pointless.
XreaL wasn't even a complete game at any stage, just an engine, and there's currently no plans for it to become anything more. And while it's engine tech will live on in ET-XReaL, there it will only be a enhancement engine to Wolf:ET rather than it's own game. It's great tech sure, but you can't compare the two as one is just an engine and one is a game. Overdose is actually a game, but it's also insane to compare as they're trying to be very different games, and one happens to have focus on high detail graphics.
The devs never made any emphasis on graphics, only emphasis on making their game. They've succeeded in what they wanted to do, and that just happens to not be what you wanted them to do. This topic of HDing up Trem has been discussed many times before, never has it appeared to convince the devs that it's something to put in the effort for.
Many fork projects that include focus on graphics have formed, and fallen apart. A current one appears to be "Dretch Storm" using XReaL/ET-XReaL for an engine. If you want HD trem, go and converse with it's patrons, as I don't think the Tremulous devs are going to be convinced anytime soon.