User blog comment:ImperiexSeed/Supernatural Mysteries/@comment-6637171-20130328120720/@comment-5767263-20130328193453

A lot of the more powerful demons are either locked away or dead. Samhain is sealed, Azazel is dead, Lilith is dead, Alastair is dead, and the Knights of Hell are all dead (except for Abaddon, who's dead in the future). Also, considering that Lilith was said to be trapped "neck-deep in the Pit" and had almost no influence over Hell at that time, it could be that all of the more powerful demons are kept in the lowest circles and can't make a bid for power. Plus, with Michael's imprisonment provoking chaos in Heaven, it's reasonable to assume that Lucifer's reimprisonment left Hell in the same state. Crowley, being the excessively savvy demon that he is, took advantage of the ensuing disarray and wiggled his way into a position of power (maybe by offering other demons a "new age" or something like that; Crowley is a really smooth talker and had a considerable amount of influence anyway due to being the King of the Crossroads and Lilith's lover). Additionally, it's possible that his "Fergus" background was just an elaborate lie, which is what the producers and even Mark Sheppard himself seem to be suggesting in light of his "Mesopotamia" remark from Goodbye Stranger.