So has there been any explanation for why the DC did not just outright conquer the OA?
The fact that there aerospace , and conventional pilots are better than the DC's was one factor, but that seemed to me to be such a satisfactory answer. We know OA worlds were conquered by the DC during the Succession Wars, so what stopped Kurtia from absorbing the whole thing(like Steiner did with the RWR)?
There are a couple of reasons.
First, did the DC conquer any of the OA worlds during the Succession Wars? I know they took some during the Reunification Wars, but IIRC, the border is pretty static, atleast from a quick glance at the maps in the various Succession Wars books. I suppose some conquering might have happened in the 3SW.
Second..what's Kurita going to get out of it? The OA has been dying since basically the Amaris Crisis. Its not like they're going to get a planet with a robust, thriving population with a decent technological level. In most cases they're going to get a planet that can barely support itself. And they're going to get dozens of these. There will be a few exceptions, but for the most part, they'd hardly be worth the effort. The one exception is going to be the mech/fighter factory in the OA, and those are already selling to the DC.
The Dragons abhors weakness in its enemies. During the 1st Succession War, Kurita turned away from its early victories against the Lyrans because it was too easy. They like fighting the Davions because they fight well and thus more honor in the victories. Not much honor to gain while fighting against enemies that can't really defend themselves.
Where are they going to get the troops? During the Succession Wars, except for the 4th really, most of the conflict has everyone easily matched. Sure, the DC has garrison troops in its interior, but they're there for a reason, and you're looking at pulling supplies and JumpShips from the front lines to fight an enemy that doesn't get you anything, except more to garrison.
The OA survives (inasmuch as the OA's existence during the Succession Wars is considered survival) for a simple reason. Its not worth the effort to conquer it.