Well, Zach (the angel who I can never remember for sure but I think is called Zachariah -- the older white guy) said that only Dean can defeat Lucifer. So, I guess they could have waited to haul him out, except maybe they were worried he wouldn't want to help if he was in Hell too long? that maybe he'd join Lucifer's side? But they needed Sam to start the war and Dean to win it for them.