"Transformers: Age of Extinction" opened in theaters on June 27. Reuters

It was no summer of love for multiplexes. Despite the success of heavy hitters such as Disney’s “Guardians of the Galaxy” and Paramount’s “Transformers: Age of Extinction,” annual U.S. box office revenue was at its lowest level in three years, tracking firm Rentrak said Monday. Ticket revenue was down more than 5 percent in 2014, to an estimated $10.35 billion, compared with $10.92 billion last year.

“The fact that we only were down 5 percent is a testament to the strong first quarter,” Paul Dergarabedian, Rentrak's senior media analyst, said. Much of this year’s drop can be pinned on what can only be described as a dismal summer, when box office revenue dropped 15 percent to its lowest total since 2006. “Summer was really down, to say the least,” Dergarabedian said.

A summer swan dive would be enough to doom the movie industry in any year; summer box office proceeds account for as much as half the film industry’s annual revenue. Dergarabedian said a number of factors conspired to produce the low turnout, from the World Cup to Fourth of July falling on a Friday, which likely hurt film attendance for that crucial holiday weekend.

Yet the low numbers are indisputably part of a trend. After posting just two year-over-year declines from 1984 to 2010, box office revenue has declined in three of the past five years. Part of this may be due to demographic shifts. The most recent statistics published by the Motion Picture Association of America show the number of frequent moviegoers, which the MPAA notes account for half of all U.S. movie ticket sales, has declined in a number of key demographics, including 18- to 24-year-olds (down 17 percent) and 25- to 39-year-olds (also down 17 percent).

“You're losing that younger audience because they're agnostic about how they get their content,” Dergarabedian said.

Hollywood did nothing this year to deviate from its recent strategy of leaning heavily on big-budget titles based on established franchises. Six of the year’s 10 top-grossing films were sequels, and all 10 were built on foundations of established brands. These ranged from films about popular toys ("Transformers: Age of Extinction," "The LEGO Movie") to a number of Marvel Comics characters ("Captain America: The Winter Soldier," "Guardians of the Galaxy"). Back in 2011, every single one of the 10 top-grossing films in the U.S. was either a sequel or a spinoff.

But instead of seeing this reliance on established brands as a problem, analysts see it as the opposite. Industry analysts expect 2015 to see a big rebound, and they expect titles such as the film adaptation of "50 Shades of Grey," new Pixar movie "Inside Out" and others to provide much of the lift.

“If you could build the perfect box office beast, you would write down this lineup for 2015,” Dergarabedian said. It's the most cyclical business of all,” he added. “It's based on a product that changes every week.”