In response to a quesiton by Jeffrey Schmid, I have no idea why Kershner didn't do ROTJ, but I assume its for the same reason that Lucas couldn't have his long time pal Steven Spielberg do it - because at the time Lucas was having a dispute with the director's guild over how the credits rolled in his film [or something like that] and so he couldn't sign an American director; he eventually settled for Richard Marquand, after being turned down by famed Canadian horror/sci-fi director David Cronenberg and the "Dune" director David Lynch.

Richard Marquand, I'm sorry to say, died a few years after he made "Return of the Jedi".