It took Warner Bros. the better part of a decade to get The Flash up and running. After losing multiple writers and directors, IT helmer Andy Muschietti seemed like a good fit for the job; excitement for the movie also increased when it was revealed to be a loose adaptation of Flashpoint featuring the return of Ben Affleck and Michael Keaton's respective versions of Batman and Supergirl's long-awaited DCEU debut.
Original plans called for the movie to set the stage for Crisis on Infinite Earths, with Affleck's Dark Knight trapped in another reality and Keaton replacing him in the DCEU ahead of the since-scrapped Batgirl movie (Sasha Calle, meanwhile, was going to take Henry Cavill's place). Alas, as regimes changed, so did the ending.
Cavill's Man of Steel was inserted into The Flash's final moments after his return in Black Adam, only to be removed when DC Studios was formed. In fact, the entire thing was then heavily reshot, with that big Crisis tease swapped for a random George Clooney cameo and a badly CG'd tooth falling out of Barry Allen's mouth.
The rest of a movie was a mess too, with abysmal visual effects, a lousy story, and not one but two terrible performances from lead star Ezra Miller (they/them). Oh, and we surely can't forget the fact the actor made headlines upwards of a year before The Flash's release for repeated alleged assaults and grooming of minors. That, combined with a terrible run for DCEU movies resulted in the movie bombing at the box office. Warner Bros. tried everything it could to turn the tide, enlisting ringing endorsements from James Gunn, Tom Cruise, and Stephen King, but none of them could turn the tide and The Flash grossed only $271.4 million worldwide on a budget believed to have ballooned to upwards of $300 million.
In a recent interview, Muschietti said this about why he believes The Flash failed:
"I think failed in the sense that it didn’t appeal to all four quadrants of the audience. When a $200 million movie is made, the studio expects to bring everyone, even your grandmother, to the theater. And in private conversations later on, I learned things like how a lot of people weren’t interested in the Flash as a character. Half of those four quadrants - the two female quadrants - many women didn’t care about Flash as a character. These are things that worked against the movie, and I gradually found out about them. But I’m very happy with the movie, and I highly recommend it."
The filmmaker also blamed Miller's "mental health situation," the "public relations crisis" that followed, and "fatigue with the superhero genre."
What isn't adding up about Muschietti's argument is his belief that "a lot of people weren’t interested in the Flash as a character." While the Scarlet Speedster certainly isn't as well-known as many other DC superheroes, he was also the star of a hugely popular CW TV show which lasted for the better part of a decade. While ratings declined in The Flash's final few years - the norm for many series which perhaps stick around a little too long - it being on air for 9 seasons somewhat dispels the notion that no one is interested (The CW also had a large female audience).
So, yeah, Muschietti's excuse is laughable at best, and dishonest at worst. The Flash failed because it was a bad movie; fans and regular filmgoers alike had lost faith in the DCEU as a franchise and only a truly great movie could restore some of that. The Flash wasn't it. It's not all on Muschietti's shoulders, of course, as a big part of why the movie flopped can be traced back to Justice League helmer Zack Snyder. Miller was a terrible casting choice for Barry Allen and not the right person to lead their own franchise. Jason Momoa wasn't the best fit for Aquaman either but he had enough charisma and star power to turn his first solo outing into a $1 billion hit...Miller did not. Not to get too much into the weeds with fan casts, but go back a decade, and people weren't wrong when they put forward names like Ryan Gosling and Bradley Cooper for the live-action Barry. A B-List character needs an A-List star, not an indie actor with no blockbuster experience.
The audience isn't responsible for The Flash's failings, but Muschietti, Snyder, and Warner Bros. are. Now, DC Studios is "holding a beat" on the character's DCU development, and surely the best option will be to shift the spotlight from Barry Allen to Wally West. For now, the former has been damaged beyond repair. As for Muschietti, he claims to still be in line to direct The Brave and the Bold for DC Studios, a shockingly poor decision from Gunn which doesn't bode well for the DCU's Batman.