Sometimes, despite however much talent and acclaim a director has earned, they're just not the right fit for a certain film.
Take Michael Haneke, for instance. He's a legendary auteur, won an Oscar this year for his film Amour and has created a reputation for his unforgiving and unsentimental films. Which is why the thought of him directing a big budget sci-fi effort like Star Wars is an automatic punchline.
For the recent Cesar Awards, the French academy put together a taste of what a Haneke-directed Jedi adventure would look like. Calling back to Amour, a dying and senile Darth Vader is driving his caring son, elderly sad sack Luke Skywalker, absolutely nuts. Even more depressing is Princess Leia; she certainly has seen better days.