The majority of the movie takes place underwater, and we don’t really need to see Batman break out the dive suit to come help out.
So in this case, I think budgetary constraints may prove to be a good thing for the overall story.
The Marvel model has basically conditioned us to now expect crossovers and cameos in comic book movies, to remind us that the film takes place in a larger cinematic universe.
But just because that is now commonplace doesn’t mean that it is always necessary or even beneficial.