Great question — answering as the UphillTech team:
There are actually two questions here - What caused the film's initial slippage and why did they decide to bump the film by almost a year. What caused the original slippage? We don't know and the studio aren't admitting anything other than issuing a bland statement about wanting to "fully realize" the film's "complex" nature. Reading between the lines, I think we can assume that the film was overly complex and needed a hell of a lot of editing in order to make it watchable by a mass audience. This is DC's tentpole film and potentially the start of a franchise worth 10-15 billion dollars. My personal guess is that that's the sort of thing you don't want to muck up. Why delay for a whole...
Full answer by user7812: https://movies.stackexchange.com/a/45297 (CC BY-SA 3.0).
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