In getting to the answer, Derek Thompson shows how it wasn't always that way:
[T]he first instances of what film archeologists would actually call "movies" around 1910 featured different prices for different films. Movies were priced according to their length, stars, and popularity. For three decades until the 1940s, one theater would have the rights to each movie within a certain zone, and movies received grades (A, B, or C) that corresponded with ticket prices at those theaters. If the rules of the 1920s ruled today, Mission Impossible might be $15 and Young Adult might be $7. What changed?
Everything. For starters, the famous Paramount anti-trust case broke up monopolies between producers and distributors. Multiplexes replaced single-serving theaters. A recession after World War II coincided with the popularity of television to gut studio revenue, forcing them to rely on fewer, more expensive movies.
Tyler Cowen rephrases the question:
Especially in the days of robust DVD sales, why did they not offer first weekend modest coupon bonuses — as distinct from price discounts — for the most popular movies? That would drive up attendance, without damaging the gross (as a lower p would), and boost "advertising" for the DVD and the subsequent foreign openings.
The related thread on whether movie theaters can survive here, here, here and here.
The Dish 