Because trailers are made before the movie is finished. This also explains differences in CGI, etc.
Most of the scenes in the first trailer for Rogue One for instance aren't in the movie, because the movie underwent a significant rewrite while in production and IIRC about 50% of the released movie is footage that was the result of those changes.
WRT Justice League, as explained on Wikipedia:
In May 2017, Snyder stepped down during post-production of the film to
properly deal with the death of his daughter. Joss Whedon, whom Snyder
had previously brought on to rewrite some additional scenes, took over
to handle post-production duties in Snyder's place. In July 2017, it
was announced the film was undergoing two months of reshoots in London
and Los Angeles, with Warner Bros. putting about $25 million into them
(more than the typical $6–10 million additional filming costs).
As you can see, the movie changed directors during post-production, and a significant amount was spent on reshoots and new scenes. By that time trailers had already been released...