Though it was far from a sure hit when released, Marvel Studios struck gold with 2008's Iron Man, the first installment in the Marvel Cinematic Universe. Director Jon Favreau's comic book movie was a huge hit, earning overwhelmingly positive reviews from critics and fans, while also thriving at the box office. Iron Man's success set the tone for the Marvel Cinematic Universe as it continued to grow - if it ain't broke, don't fix it. No matter how dark a MCU movie's story may get, there was always a focus on sprinkling in plenty of comedy while also trying to appeal to fans of all ages. MCU movies built a massive fanbase over the years and its movies continue to receive mostly positive reactions across the board.
Warner Bros. didn't have the same kind of luck with its cinematic universe, which is often referred to as the DC Extended Universe.
Zack Snyder's Man of Steel, which launched WB's DC universe in 2013, received mixed reviews from critics, and fans still have heated debates about its handling of DC's most noble hero. Snyder's follow-up movie, Batman v Superman: Dawn of Justice, received mostly negative reviews from critics and once again left fans divided, and just like Man of Steel, fans are still debating the film. Some expected David Ayer's Suicide Squad to be the first DCEU movie to receive a positive reaction from critics while also uniting fans, but it was subject to very negative reviews as well, proving to be yet another polarizing movie among fans. These movies generated money at the box office, yes, but they also generated mockery (MoS & BvS' tone; Suicide Squad's character designs), and WB couldn't win over the critics.
Even though Patty Jenkins' Wonder Woman featured some dark elements (like an entire village being killed by poison gas), the movie also included lots of comedy, and was the first DCEU movie to be praised by critics and fans. Finally, a DCEU movie received mostly positive reactions - and, many fans pointed out, it felt more like a MCU movie.
Then came Justice League, which should have been the biggest DCEU film to date. But the anticipated blockbuster generated the least amount of money from the DCEU slate of movies, and this is despite the movie having more humor added to it via Joss Whedon's reshoots. The first live-action Justice League movie underperformed, a massive disappointment to the studio as well as the countless fans who waited for years to see a live-action version of the team on the big screen. The future of the DCEU was uncertain, and if the studio wanted to make critically-acclaimed movies and generate mostly positive reactions from fans, changes needed to be made... and changes most certainly have been made.