Its' simple it was too damn much. Come on, while he is an Oscar winner and box office star he is human. He has multiple projects plus family life going on.
Being the actor, director & writer in the cornerstone movie of the DC Cinematic Universe was a lot of pressure. Not to mention, his expected appearance in other DC films whether cameos like his in Suicide Squad or part of the team in the upcoming Justice League.
Not to mention his non-DC projects.
Ben did the right thing. He stepped back from his role as director so that hat he could finish a killer Batman movie script. Being a director means you have to wear a lot of different hats. By giving up the seat, I do not expect Ben to give up what vision he imagined for the movie. Instead, he will helped to pick a director who will work to have his vision realized on film.
Check out the reaction.
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Kudos to you Ben.

Spinnerrack---Out!!