User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-170.235.245.52-20130523161328/@comment-5927468-20130524025814

Maybe he takes a long time to reform. Even with the Doors of Death open, it might be that some monsters need time to get their strength back. In BotL Eurytion says that Geryon wouldn't be back for a century. Maybe Antaeus is the same way. Of course it could just be that RR didn't want to recycle that particular enemy