I always figured that it went down like this as they weren't sure if Han survived the trip. Sure, Lando said he was in perfect hibernation, but there was no gaurantee that Jabba wouldn't thaw him out and kill him.
Leia's role was to thaw him out and make sure he was all right, then high-tail it out of there and send word to Luke.
When that word didn't come, plan B went into effect where Luke came to rescue them all.