....because music doesn't matter if it belongs to canon or not.
I feel like that's subjective.
Generally, if I were to be playing a mod about a separate version of a universe that already has an official interpretation to it, I wouldn't think I'd like to hear the official interpretation's music. It wouldn't fit, especially since I know the source of where it came from. Some new, custom (and possibly fan-made) music wouldn't hurt Imperial Civil War, though. I'd say it'd have to be in Empire at War's "general ambiance"-style, but that's besides the point. It would also be a heavy chore making special music for every faction.
I wouldn't mind some "inspired" music of the canon interpretation, though...