I'm not sure where this site is drawing its information. That could explain any discrepancy you and I are seeing with the numbers.
http://mrob.com/pub/film-video/topadj.html
I think other entertainment streams heavily affects movie ticket sales. Anything released before 1985 or so had to be seen in the theatre or you're stuck waiting for it to air on television. After that, VHS took over and the home theatre market was born.
On the other hand, you also have to factor in the US' much larger population, over 310 million now. Compare that to the 220 million in 1977 and you realize just how many more people can go to the theatre to see a movie now versus the 70s or early 80s. It's close to 50% more people.
So, I think it's a factor of all these things.
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