Before the Ori were destroyed the Asgard power gave a unique energy signature so it could be tracked. I’m assuming after Ark of truth they started using Asgard power cores again.

Well its mostly because the Asgard data hasn't really been fully decoded and integrated into Tau'ri technology when the war against the Ori was on going, plus, the Asgard supercore gave off a unique energy signature the Ori could track.