The answer to this is simple, Han dies from being placed in carbonite inside of Jabba's palace. It's one final sadistic joke for Jabba the Hutt to experience for someone who betrayed him. Lando takes his place down on Endor and someone else takes the helm of the Millennium Falcon to destroy the Second Death Star, probably Chewbacca, in a way that would avenge the death of Han in part at the hands of the Empire.
Well, I think there's more to the question than that. Lucas clearly wanted Return of the Jedi to be a lighter, more toy friendly, film, than either of the films that preceded it. That instinct is why Han wasn't killed off, despite elements of the creative team advocating for such an outcome. Here Han has to die, and that makes the film darker than Lucas wants. Also, Han's death raises the question of how the romantic subplot works out, given that a decision had clearly been made in Empire for Han and Leia to be the couple, even if the secret sibling idea had not been finalized then as the resolution. Here, Han is dead, so that situation is a little more complicated. Rather than being siblings, Luke and Leia might end up together, or they might both end up alone. Even if Lucas was planning on the siblings plot by then, he could change his mind. After all, that sort of thing happened repeatedly with Lucas and his creative team.
Beyond that, there's the question of Harrison Ford's career. If he makes the decision late enough, he can still be in Raiders of the Lost Ark. But not returning to Star Wars could result in tension between Lucas and Ford, to the point where Raiders may not have a sequel, or the role of Indiana Jones might be recast. Either of those options would have an impact on Ford's career in the 1980's. Also, there's the question of what Ford would do instead of the third Star Wars film.