We know that planets were golden 600 years ago and since scourge came later i say there was a massive war that royally screwed over the cluster. The Jaardan built the vaults to repair the cluster but the other side was having none of it and as a final measure dropped the scourge. I mean Jaardan use way too much automation for their tech and the other side basically used a cluster buster on them. So the war royally screwed over both the sides and both did whatever they could to gain an advantage with what they had. The remnants probably wanted planets where they could build up again and the Angara were going to be a slave/clone army to continue the fight. The other side used what they had ie Scourge to target remtech specifically.
The above is the first answer the second is something i hate but would not be surprised. Organics vs synthetics, the remnants were going more synthetic in their tech and the other side wanted the organic supremacy (forefathers of the kett culture perhaps). That was that was the cause of war while Angara were meant to be enhanced with cybernetics clone soldiers, their designed bio electricity ability to power the VI or AI enhancements.
