All we can go from is canon. In canon, the Sath was apparently traveling relatively alone, and appears to have had a lead-in force upon leaving the Nebula of only a few-dozen fighters(A Flaming Sword). Presumably, the Beleth was trailing behind somewhat.
Canon also suggests that there were no reinforcements held in reserve to assist the Sath, to speak of(of course, it might just mean the Shivans had nothing big enough, and/or had only cruisers that they were unwilling to waste throwing up against the Colossus, at the moment).
Arguably, there are a few plot holes here(failing to destroy the Beleth doesn't cause it to reappear in High Noon, and leaving some of the Sath's main beams alive in Bearbaiting allows it to destroy some "line of defense" the Alliance had set up in Capella, which isn't mentioned in briefings when all the cannons are destroyed); the former perhaps suggesting the Shivans really don't care at all about the loss of one Sathanas, which is perhaps relevant.