Return of the Jedi

Lawrence Kasdan wrote the scripts for The Empire Strikes Back and Raiders of the Lost Ark, which both stand out for having the best dialogue in their respective series. But he also wrote the script for Return of the Jedi, which is plagued throughout by clunkers. What happened?

Wikipedia explains:

Unusually, the screenplay itself was not created until rather late in pre-production, well after a production schedule and budget had been created by Kazanjian and Marquand had been hired. Instead, the production team relied on Lucas's story and rough draft in order to commence work with the art department. When it came time to formally write a shooting script, Lucas, Kasdan, Marquand, and Kazanjian spent two weeks in conference discussing ideas; Kasdan used tape transcripts of these meetings to then construct the script.