Probably has something to do with the script.
First thing I thought of. Or they could be wanting to do a two for one deal. Film Man of Steel 2 and the first Justice League film together if MoS2 was suppose to lead into it. The word was that a number of cameos were suppose to be in the film. It would be better to just everything together and they would need extra time to plan that out I think.
This delay is disappointing though. I wanted to see Batman vs Superman/Man of Steel 2 more then any other film in 2015. Even more then Star Wars.