The Ancients were intelligent people, but at the basic level, they have no clue.
For the cry out loud, the Asgard did ask the Tau'ri for help with the Replicators.
Because as Thor said, they need less intelligent solutions to stop the Replicators.
Look at this scene. If the Wraith did attack the Ori. https://www.youtube.com/watch?v=UNeS7qMMvbw