Basically you can blame MTV.
It became a pretty open secret that the average moviegoer's attention span is dropping, this would have been in the early '90s.
Since someone came up with the equation ADD = $$$, all the studio execs starting using running time as factor.
Someone, I forget who, did a study and found that the average viewer will watch three episodes of something on television in a row, but not neccessarily four in a row. The younger the audience, the less likely. Since tv viewers = movie viewers, well you can figure it out from there.
Personally, I think it's crap. I'm not bailing on seeing a movie because it's 2 hours and 12 minutes instead of 2 hours. And I think 90 minutes is the biggest joke of a running time ever. Three 30-minute sections for the beginning, middle, and end? How formulaic and boring can you get?
It's especially killer with auteurs like Scorcese or Ang Lee. Those are the kind of cats you're gonna sit through 3 hours if need be, they're storytellers. But given the state of movies today, most movies aren't worth the attention beyond 2 hours. A lot of it does come down to the fact that filmmakers aren't good at holding your attention.
Hope this provided some insight on the matter.
PEACE


"Your current frequencies of understanding outweigh that which has been given for you to understand." Saul Williams