The old "you're only as good as your last movie" argument, eh? ;)
Kershner made a little thriller called "Eyes Of Laura Mars" which probably got him the Empire gig. He also directed what is probably one of the best episodes of Speilberg's "Amazing Stories", the side splitting "Hell Toupee".
There's a persistent illusion that a film director is lord of all they survey. They are often simply a gun for hire. One can be saddled with an oft rewritten script, and micromanaging producers breathing down your neck. (Not to mention prima donna actors!) And then there's the issue of whether you get "final cut".
Kershner got an amazing free rein on Empire, that Marquand probably never had on Jedi. It's accepted by many fans Lucas was backseat directing on that, so it's likely to have happened to Kersh in his long career on other films.
Based on what I've read in "Once Upon A Galaxy", the guy knows how to make a movie, given the right circumstances. The fact we're still marvelling over Empire 30 years later is proof.