According to sources who spoke with Collider, 20th Century Fox recently ran a test screening for X-Men: Dark Phoenix to see what was working and what didn’t.
The earliest the main actors could all reunite was in August or September, which would have only left a couple months to perfect the visual effects.
Not wanting to rush the work, Fox decided to push Dark Phoenix to next February so that there would be enough time to finish the reshoots and post-production with some time to spare.
However, in the case of Dark Phoenix, many members of the cast are busy with other projects; particularly Jean Grey herself, Sophie Turner, who is busy shooting Game of Thrones’ final season.