Yeah, I wondered that too. Within our "sector" (so to speak), the Thraddash were the first local species to be conquered by the Ur-Quan. Their fleet should have been brimming with vessels from other areas, and their Dreadnoughts filled with enslaved crew members from the same.
Only thing I can figure is that they don't bother calling in the extra numbers unless the situation was dire. I imagine if Zelnick had shown up with a massive fleet of Avatars instead of doing the sneaky "blow up the Sa-Matra" plot, the Ur-Quan may have eventually called in reinforcements.
Hell, reinforcements from another sector has great potential for a sequel. Say we take the Warlok Files version of the post-SC2 era. Ur-Quan have all but vanished as a fleet, you only encounter them as single vessels. But meanwhile, the Ur-Quan you *don't* see are busy speeding between the various conquered sectors of the galaxy, using the authority they've built up over the past 20 millennia to press various fleets back into active "battle thrall" service.