User blog comment:TB93/The Prince of Hell/@comment-3481504-20150313231340

As with most, if not all, hierarchies in Supernatural, the strongest individual of any given group/species, is the one who becomes the authority of its peers. So, whether or not Gavin would become King of Hell after Crowley's death would depend on how powerful he is compared to all other demons. The only reason Crowley ascended to the throne and not Samhain is probably because Samhain is actually leashed in Hell, and only the ritual can release him.