The main driver IMO is the death of the tight 90 minute, 80 Million decently acted thriller / action / comedy film.
Everything is too big, too epic, too simplistic, and too long.
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...
Often during a three hour film I've ran out of refreshments and would like to buy a drink or something for the last hour.