If I understand movie theater economics correctly, the studio gets 80 to 95% of the ticket sales, depending on how "first run" the movie is. The theaters actually make their money on selling concessions.
Well, the longer the movie, the more people feel the need of snacks to get through it. So maybe the theaters are pushing longer movies rather than shorter, because they make more money that way.
Just an off-the-cuff hypothesis...
