According to Joseph Gordon-Levitt, he just couldn't work with the folks at New Line Cinema. He explains it all in part of a very long Facebook post:
Recently, as you also might know if you like to follow these sorts of things, the sorta "ownership" (for lack of a better term) of the Sandman material changed hands when Warner Brothers shifted the entire catalogue of Vertigo comics (an imprint of DC) to their subsidiary, New Line. And a few months ago, I came to realize that the folks at New Line and I just don't see eye to eye on what makes Sandman special, and what a film adaptation could/should be. So unfortunately, I decided to remove myself from the project. I wish nothing but the best for the team moving forward.