When you keep releasing a slew of poorly written movies, yet continue to demand unreasonable fees, this is the result. People aren't willing to shell out the bucks to see a B grade movie. It's just not worth it anymore.
This gets mentioned a lot on Slashdot but, in reality, the number of "good" movies has remained reasonably unchanged each year.
Here are the movies in the IMDB Top 250 grouped and counted by year:
In fact, 2014 (Interstellar, Boyhood, Gone Girl, Guardians of the Galaxy, The Grand Budapest Hotel and X-Men: Days of Future Past) was actually a better year than 2013 (The Wolf of Wall Street, Rush, 12 Years a Slave and Prisoners).
The "prime" year was 1995 but that only resulted in 10 films (Se7en, The Usual Suspects, Braveheart, Toy Story, Heat, Casino, Twelve Monkeys, Before Sunrise, La Haine and Underground).
(Nitpickers will point out that I really should run this over the entire DB and not the Top 250 and all take into account all film ratings - they'd be right, but that's a lot more work which I don't have the time to do).