It's not, necessarily. We're all just speculating here.
Could just be like proof-of-concept, or he knew/acknowledged he's taking longer than the other writers in the DC 'verse have so far and wanted to prove he's at least plodding away at it slow-methodical-and-steady. Have something to show the suits, maybe impress them and set them more at-ease.
In any case, after the two Apes movies and Let Me In, I'd say he's earned the benefit of the doubt as far as the way he works.
If DC's happy to leave the guy at his own pace, even maybe delay the release a little if they have to, that can only be a good thing. It sounds like they trust the guy.