User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-3498371-20130809193818/@comment-3146930-20130809194406

I think it is because without Percy, they don't want to risk flying over the sea. I mean the Argo II has been pretty beat up during The Mark of Athena, so if something went wrong and they had to land in the water, the wouldn't have their sea expert for help. I guess with Hazel and Nico on the ship, they feel safer flying over land then sea.