It's the impatient... now now now mentality that has propagated through our lives. The notion of putting the big movies on 3 or 4 screens in a single multiplex... plus the size of the multiplexes makes it so everyone can see the movie as soon as they want. There is no need for a movie to run for 8-10 weeks because the excess demand, combined with the front-end loaded advertising makes for a boom or bust 1-3 weeks.
Remember when you used to go and find the movie 'sold out' and had to pick another movie? I have to imagine theatres are actually losing money compared to the old model.. where people would see their second choice, and then come back another night to see their first choice.
Now with internet ticketing and so many screens.. you never show up to find yourself shutout.
Theatres have kind of whored themselves out... there is no more drawn out relationship.. just hit it and quit it