Answer by Mildred Lewis:
As cinema ages as an art form, it is facing formidable challenges from video games and television, particularly quality television, e.g., Breaking Bad, Game of Thrones, House of Cards, The Wire.
How do movies compete? By offering an experience that is distinct from what can be seen in a home theater or on a tablet. Hence, there are enormous artistic and commercial pressures pushing movies toward spectacle and greater visual and narrative scale. These pressures tend to favor longer films.
Competing with quality television might also push movies toward greater length. Quality television has basically replaced many indie and women oriented films. Consider that you are much more likely to see engaging, powerful roles for women, especially women over 30, on television than on film. This is a long standing trend, cf. Gena Rowlands' career. One of the pleasures of television is that it gives audiences the opportunity to spend a great deal of time with characters. To compete, films may be trying to offer a comparably immersive experience.
I also have a suspicion that the increasing reliance on international markets has something to do with this as well.