Originally, 2018 was scheduled to mark the first time Warner Bros released three films in the DCEU in a single year, but that continues to look unlikely. The Flash was thought to kick off the year in March, but is currently searching for a new director; Aquaman should have been second, but now appears to finish off the year; and The Batman was thought to be taking the October date, but clearly that is not happening anymore.
Hopefully the delay is because Warner Bros wants to give Wan more time to see his vision completed and avoid some of the other reported troubles they’ve faced. WB could have also made this decision because they are still figuring out what the plan for the universe is post-Justice League and want to make sure Aquaman will fit into that continuity.
Seeing a movie get pushed back for unknown reasons is usually cause for concern, but one reason why this may have happened is due to something completely out of WB’s control. Paramount recently put Mission: Impossible 6 on Aquaman‘s previous release date, and given the success of Mission: Impossible – Rogue Nation and the draw of Tom Cruise, it is a smart move to not go toe-to-toe with another major movie franchise. Now, Aquaman will avoid the competitive months of the summer season and (if good) be able to be one of the bigger releases for the few weeks surrounding its new release date.