User blog comment:B214/High School DxD New Episode 3 Discussion/@comment-6482575-20130725093346/@comment-1359322-20130725102130

I think this is already explained in the series, the Great War caused too much damage to the three factions to the point that all 3 factions lost majority of their forces and without the leaders of the Angels and Devils (Azazel has ordered the Fallen Angels to retreat), they decided to stop the war. So that is how some of the 72 Pillars survived because they decided that they couldn't continue the war after the Four Great Satans and the Biblical God died.