After three successive weeks topping the U.S. box office — it finally dropped to No. 2 over the Labor Day Weekend, behind “War Room” – the rap drama has made more than five times its estimated $28 million budget. It helped Universal break a speed record for passing $2 billion in domestic ticket sales in a single year, has become the most successful music bio in history, and stands at No. 10 on the list of 2015 earners, with over $150 million in domestic sales as of Monday.

Those numbers took many by surprise (the film’s $60 million opening weekend was well above early predictions of $40 million). But that surprise has given way to analysis – and a sense that the film’s success isn’t just understandable, but was in some ways inevitable. Experts point to five factors:

Timing: The summer movie season has grown increasingly front-loaded, with May and June crowded with would-be blockbusters. By August 14, though, when “Compton” opened, the decks were clear. “All the heavy hitters were out of the way,” says Gitesh Pandya, editor of Boxofficeguru.com. “That whole wave of action sequels, superhero films had finished.” “Compton” offered something different, too. “It’s something of significance but with broad appeal,” says Nick Carpou, President of Domestic Distribution at Universal. “Especially for audiences who might have gotten tired of more traditional popcorn films and want something to sink their teeth into.”

Savvy marketing, followed by strong word of mouth: The studio was smart and pushed the film with recording-industry tie-ins and social media; a “Straight Outta” meme, featuring stars like Beyonce, took over the web. But when people did actually see it, the movie was greeted with generally positive reviews – the film scored 77% among “Top Critics” on Rotten Tomatoes – and a strong audience reaction. “You can’t make people like a terrible movie,” says Paul Degarabedian, senior media analyst at the audience-measurement firm Rentrak. “If it had terrible reviews, terrible social-media buzz, it could not have held onto that No. 1 spot. But people have wanted to see this since the first teaser trailer back in April. And when it came out, it delivered – 75% of audience members said they’d definitely recommend it.”

Cross over appeal: The biggest mistake some analysts may have made was seeing the film’s audience as exclusively young, African-American and male. These days Dr. Dre is a mogul, Ice Cube is a movie star and hip-hop music has become everyone’s music. “Straight Outta Compton” served an underserved audience, but it also reached way beyond that base. Rentrak’s PostTrak demographics revealed crowds that were 54% over 25, 40% female, 25% Caucasian and 18% Hispanic. “It resonated with a broad audience,” says Degarabedian. “It struck a chord.” “You could call it the urban audience, but it’s not exclusively an African-American audience,” says Pandya. “This kind of music and subject matter goes beyond race.” (Interestingly, though, the movie that finally pushed “Compton” out of its top box-office spot? “War Room,” a faith-based, African-American-centered, family drama.)

A message that resonates: Although the movie is presented as the story of the ‘80s rap group N.W.A., it also deals with the justice system, police brutality and inner-city unrest – still hot-button issues. While social-justice drama isn’t enough to sell a movie — “`Selma’ had many of the same themes, and underperformed,” Pandya points out – “Straight Outta Compton” seemed to spark a discussion. “That’s what you want art in a culture to do, to create dialogue and hopefully to create a deeper understanding of our culture,” says Carpou. And even when the dialogue grew more critical – as some writers questioned the movie’s refusal to deal with issues like misogyny or anti-Semitism – it didn’t hurt ticket sales. “Actually, a little controversy can help the box office,” says Degarabedian.

The studio that released it: Universal has been on a streak all year, with “Fifty Shades of Gray,” “Minions,” “Furious 7,” “Trainwreck” and “Jurassic World.” It knows how to build a hit. Carpou insists it’s no secret, either; just smart, supervised business. “It’s a confluence of elements – great movies, terrific filmmakers, performers who absolutely attach to their public,” he says. “It starts at the point from which the deals are made and continues to how the films are actually produced, to the way they’re positioned for a release date, to the way marketing empowers people to watch – and we’ve seen all these elements falling into place.” “Universal had 35% of the entire summer box office,” marvels Pandya. “They just did gangbusters.”

“They’ve had one of those years where they could almost do no wrong,” agrees Degarabedian. “When an exec goes to sleep at night, this is what they dream of. Unless they wake up, worrying – How do we live up to this next year?”

Stephen Whitty is a two-time past chairman of the New York Film Critics Circle. He writes for The Star-Ledger, The Daily News and other magazines and websites. Follow him @StephenWhitty