Christopher Nolan doesn't like superheroes. It's pretty obvious when you watch his Batman movies; he's constantly trying to get away from the tropes of the genre and he's at his best when the characters aren't in their costumes. And there's nothing wrong with that - superhero stuff just ain't his thing. He gets James Bond, he doesn't get Captain Atom.
Which is why it's good news that he won't be involved with the upcoming Justice League movie, which he confirmed at the junket for The Dark Knight Rises. Warner Bros roped Nolan in to produce Man of Steel, hoping that he would bring his The Dark Knight cred to the Superman reboot, but that's never felt like a natural fit. And a movie about the Justice League - a team that features a Martian, a speedster, an Amazonian princess and a guy who breathes underwater - needs to be bigger and less 'down to earth' than what Nolan does.
I'm sure Warner Bros would have liked him to come on board, but they're better off without him on this particular project. Get certain filmmakers involved with things that suit their strengths, and don't distort projects to better fit a specific filmmaker. Now if they could just get that Greg Berlanti the hell away from their DC movies maybe we'd all be better off.