Justice League really needed to do close to Avengers numbers. You're dealing with DC's trinity of super heroes that damn near everyone has known for decades. The Avengers really were just what Marvel had the rights to when they started with Iron Man and didn't have nearly the recognition as Bats, Supes, and WW.

JL, with their established heavy hitters, didn't come close to the team up movies (which I'd say established the heroes in the public mind share, not just in comic circles), or the stand alone movies. I'd go back and check the numbers of the stand alone MCU movies pre-Avengers and see how JL stacked up, and I wouldn't be surprised if they still came up short. Matching the solo movies would still have been a loss. Not even getting to that level is much worse.

In short, I'd say none of the characters in the MCU, save maybe Spidey (who may not count) had the cache of DC's big 3 before the MCU. With that kind of recognition having the 3 of them in a movie and forming the Justice league should have been sure money. It wasn't