Okay, my theory.
I think the Ancients actually were not powerful enough to take on the Ori.
The Ori had an entire galaxy of followers worshipping them, and therefore gaining more power all the time. The Ancients didn't have a lot of followers, and even the ones that did, didn't worship them quite like how Ori worshippers did.
As a comparison, a 5th season SUPERNATURAL episode called "HAMMER OF THE GODS". Lucifer, the archangel, completely shredded a LOT of pagan gods, but the only god who was able to put up a fight, or even survived, was Kali. Kali still had a lot of worshippers due to so many people following Hindu, which I believe is right behind Christianity in terms of how many people practice it. The other gods had no chance because so few people follow them.
I think this is a direct parallel to the Ori and Ancients... the Ori is Lucifer, and the Ancients are the pagan gods.
The Ori had an entire galaxy of followers worshipping them, and therefore gaining more power all the time. The Ancients didn't have a lot of followers, and even the ones that did, didn't worship them quite like how Ori worshippers did.
As a comparison, a 5th season SUPERNATURAL episode called "HAMMER OF THE GODS". Lucifer, the archangel, completely shredded a LOT of pagan gods, but the only god who was able to put up a fight, or even survived, was Kali. Kali still had a lot of worshippers due to so many people following Hindu, which I believe is right behind Christianity in terms of how many people practice it. The other gods had no chance because so few people follow them.
I think this is a direct parallel to the Ori and Ancients... the Ori is Lucifer, and the Ancients are the pagan gods.