According to Empire of Dreams, Lucas had no choice but to get a non-American director for Return of the Jedi due to his departure from the Director's Guild of America. They had excessively fined him for The Empire Strikes Back's absence of any opening credits, and because he didn't want Irvin Kershner paying those DGA fines himself, he covered for him. He knew it would only happen again if he got an American to make ROTJ, and was so impressed by "Eye of the Needle" that he asked Marquand to direct.

Now that I think about it, Lucas's departure from the DGA might have a lot to do with why he decided to direct the prequels himself.