He's a bounty hunter - as soon as he's got his money, he's off to catch another bounty and get more. Why the hell would he be hanging out at Jabba's in the first place?

That's my "in-universe" rationale. My REAL reason: He does nothing in this entire movie but die. And his death isn't even a good one - he's accidentally killed by a blind guy. He serves absolutely no purpose other than that fans liked him in ESB so George decided to put him in ROTJ.

It's better for him to show up in ESB, do his thing there, then jet off, never to be seen again. It makes him a more interesting, mysterious character than a bounty hunter who likes to hang out at some fat gangster's palace for no reason. His presence in ROTJ, like his/Jango's presence in AOTC, makes him less of a badass, because beyond his function in ESB George didn't know what to do with him other than put him in to please fans.

He shouldn't be in ROTJ, and he shouldn't be in AOTC.