The most likely explanation is that Fincher’s plate is currently very busy and he couldn’t make the planned schedule that the studio had for production on the film. He is currently hard at work on Netflix’s first original series House of Cards, which is set to debut next year and will probably take up a large portion of his time until release. He has directed the first two episodes of the first season and will most likely be overseeing the other episodes as they go through post production.
As for feature films, Fincher has talked for a long time about his own version of 20,000 Leagues Under the Sea as his next project. He is currently “working on it” but has been reticent to give any further details considering no deals have been signed on his end.
Cleopatra probably just doesn’t fit in with his prior commitments and he has decided to pass for that reason. That being said, I also have the feeling that Fincher wasn’t eager to attach himself to another remake following Dragon Tattoo, House of Cards and 20,000 Leagues and is more interested in developing original material for the big screen.
So, Cleopatra will be making it to the screen but without David Fincher. No doubt a blow for Sony but they will move on and find someone else. Ang Lee is an interesting choice but I’d also like to see who else they’re chasing for the project.
No doubt we’ll hear more on that front soon.