After blowing up Jabba's sail barge, Luke and R2 leave in his X-Wing, while the rest of the gang leave in the Falcon. But Boba Fett may or may not be getting out of the Sarlacc, and his ship is probably in the palace hangar if it has one, or else is parked nearby, probably visibly. For the amount of effort the rebels spend for every little ship, you'd think they'd take it. Or, if Solo and Fett still have a canon rivalry, maybe they'd blow it up.
It seems Lucas completely forgot about Fett's cool ship, and so did everyone else.