I don't read comic books, but love the Superhero movies, and one reason I felt the Justice League movie would fail as opposed to the Marvel Avengers movie is that with the Avengers movie, to those that didn't read the comic books and just watched the movies, they were already very familiar with most of the characters. By the point the first Avengers movie was released in 2012, there were already two Iron Man movies, two Hulk movies (one a reboot because the first one was a mess), a Captain America movie, a Thor movie (which set up the villain for the Avengers movie), and the Samuel L. Jackson character was the thread that tied the movies together. The Black Widow character was already introduced in the second Iron Man movie, and only the Hawkeye character was new to the Avengers movie. By that point Marvel had produced five movies in four years which set up The Avengers.
By contrast, DC had three movies which set up their Justice League movie, and it felt a bit rushed. Ben Affleck never had his own Batman movie, he shared one with Superman, which also introduced Wonder Woman. Wonder Woman had her own movie, and Superman had his own origin movie. In the Batman v. Superman movie, they gave quick glimpses of all the other characters who would be in the upcoming Justice League movie such as Aquaman. The Flash character was introduced as part of the ensemble from the Suicide Squad movie, but they couldn't get the guy who plays him on the TV series. DC should have first came out with a couple more origins movies for Aquaman and the other characters, and maybe Affleck should have gotten another Batman movie where he wasn't fighting Superman (maybe taking on Jared Leto as the Joker instead?) before assembling everybody for a Justice League movie, and movie fans would have been more familiar with the characters and would have turned out for it. As it turned out, only comic book fans turned out to see it.