BvS and JL writer Chris Terrio said before the release of Batman v Superman that the larger story was structured that way to make Justice League a moment of redemption.
Whatever the reason, the DCEU is clearly going through changes, starting with Wonder Woman in June, and Justice League will continue that trend in November.
Much has been made of the tone of the DCEU, especially after Batman v Superman: Dawn of Justice, which drew a strong critical backlash for its darker aesthetic and story.
The true differences between the look and feel of Justice League compared to Man of Steel and BvS won’t be clear for a few more days until the trailer finally arrives, though.