The same writers/directors combo for Winter Soldier and Civil War are doing the Avengers: Infinity War movies. Those are supposed to start filming in the second half of 2016. Marvel got themselves into a bit of a time crunch by giving dates to everything so early. And then trying to fit a Spider-Man and second Ant-Man movie into things.
And they seem to like that early May slot enough that I imagine they aren't going to let Thor or Black Panther or somebody have one of those May slots to move Avengers back, especially after they made BvS flee in terror:
Iron Man - May 2, 2008
Iron Man 2 - May 7, 2010
Thor - May 6, 2011
The Avengers - May 4, 2012
Iron Man 3 - May 3, 2013
Avengers: Age of Ultron - May 1, 2015
Captain America: Civil War - May 6, 2016
Guardians of the Galaxy Vol. 2 - May 5, 2017
Avengers: Infinity War - Part I - May 4, 2018
Avengers: Infinity War - Part II - May 3, 2019
Warner's schedule is such a mess in comparison, Justice League Part One in November 2017 and Part Two in June 2019? And they want to fit another set of Batman and Superman films in there somewhere. Everybody's films are going to have to take place before or after Justice League, while coming out in the middle of it. Marvel at least cleared the board so none of their Avengers, Guardians, Spider-Man characters have films between the two parts. And I imagine only Snyder, Goyer and Terrio know what the Justice League plot even will be considering their secrecy crazy over BvS. And they're probably afraid once they start moving films it'll be seen as demoting Cyborg, Shazam, Cyborg, Aquaman, and Cyborg to second tier status.