I don't think that's the primary reason. Batman v Superman, two most iconic superheroes ever, first time on BIG SCREEN, and Warner Bros don't think this one won't top the Cap 3 where people have already seen Ironman and Cap several times, and add to that was the failure of Age of Ultron compared to The Avengers?
They moved BvS mainly because they want to top the list of Highest Grossing Films of All Time or be in that conversation imo. They are maximizing the gross they will be gaining when the movie is released. Obviously, you'll gross more on a week without a major competitor around.
Star Wars this Christmas will probably obliterate any chance of that. It could join the Avatar and Titantic club in 3 billion in theater and blue ray sales. Not to mention how much will be also generated for Disney via cable and dish on demand, online streaming services like Netflix, merchandising, and theme park tie-ins.
BvS will be hugely successful, but no way it gets close to the 2-3 billion club. Star Wars on the other hand.....