Well, maybe it's because of the relationship that Jabba had had with the Old Republic, what with their Jedi Knights saving Little Stinky and all, that Jabba's loyalty would lie with the Empire that took the Republic's place. Therefore, pressure from the Rebellion would mean nothing!
I know, that doesn't make any sense, but neither does the rest of this!
But here's something that does make sense. I said earlier that Lando could have simply unfrozen Han. Has everybody forgotten that he would also have been the most obvious, not to mention qualified, choice? After all, it was Lando's freezing chamber that put Han away, so surely he would have know the most about safely deactivating it.
Come to think of it what did Lando do besides pull his face mask down so he could more easily see out of his mouth, and then later have to be rescued by Han, the guy Lando was supposed to be rescuing? Good thing that guy blew up the Death Star...