When Jason Momoa's Aquaman debuted in last year's Justice League, he did along the rest of the superhero team. So right out of the gate, Aquaman was woven into the fabric of the larger world opposite these other heroes. That being the case, you might expect that for his first solo film, other major heroes would be showing up at least for cameos. However, that will not be the case in James Wan's Aquaman. So is the lack of cameos some story point we can read into? Nope, turns out that it's because superheroes charge hefty appearance fees. Jason Momoa explained why Aquaman won't have major DC cameos, saying:
I think it would cost a lot of money, probably. I don't think we got it in the budget. But, I think it would be great. At some point, I'm pretty sure, y'know, if this does well and if that does well...Probably, the same thing that happened with Marvel, you know what I mean?
You might not expect this level of honesty to this question, but speaking to CinemaBlend's own Eric Eisenberg and other journalists during the Aquaman set visit, Jason Momoa is quite matter of fact about the reality of the situation. Simply put, the more big name actors who have parts in the film, even cameos, the more it costs. Ben Affleck, Henry Cavill and Gal Gadot don't come cheap. These movies don't have unlimited budgets, and perhaps for the folks at WB, DC and James Wan, they felt that money was better spent elsewhere. After all, two-thirds of this movie takes place underwater, which is a huge amount. I'm sure that those effects aren't easy or cheap and having underwater scenes look good is critical to a movie called Aquaman.
This doesn't mean that these kinds of cameos can't happen in the future of course. Jason Momoa notes that future cameos are presumably predicated on the success of Aquaman. Just like Marvel, box office and critical performance allows for higher budgets, which in turn allows for more cameos as the DCEU grows. So should that happen, Jason Momoa is hoping some of his superheroic buddies might pop up in a sequel.
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. This is Aquaman's story and the focus should be on him and Atlantis. 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. Aquaman was already established as a part of the wider world in Justice League, so better not to shoehorn in cameos and just let Aquaman's story play out on its own without too much focus on universe-sharing. So in this case, I think budgetary constraints may prove to be a good thing for the overall story.