User blog comment:Kajune/SPN's redeemed antagonists/@comment-26271908-20160601191551/@comment-34326521-20160602220844

In-universe, it makes more sense for Lucifer to take Castiel as a vessel because Castiel's vessel is very strong. Millions of souls and several hundred Leviathans only started to take effect weeks - maybe months - after they were absorbed into Castiel, and it took a lot longer before they almost did irreversible damage.