According to Birth.Movies.Death, the Conjuring director is "feeling a tremendous amount of trepidation" about entering into the studio's DC Comics-inspired movie universe. As they cite unnamed multiple sources, it could just be a gigantic game of telephone or the filmmaker, known for his horror franchises and Fast 7, may be reconsidering the hassle of tentpole action films and the burdens of superhero movie making.
Wan's films like Saw, Insidious and The Conjuring are all modestly produced, but big earners. They also reflect an older style of filmmaking he is known to prefer. As recently as WonderCon the director said "my aesthetic tends to be classical." An Aquaman feature would presumably share some of the highly stylized and computer-assisted visual look established by Zack Snyder in Man of Steel and Batman v Superman: Dawn of Justice.
During his WonderCon appearance, Wan also said he expected the challenge of a superhero movie to mirror those of his smaller horror productions. "You still need to be creative. You may have the best tools to make it work, but you need characters and story," he explained. "If you have that, you can put them in any situation, big or small."
But Fast 7 was a big production and was known to be a physical strain on Wan despite wanting the challenge of a large, complicated film. During an interview with The Daily Beast at the time of the film's release, he said he was looking forward to returning to The Conjuring as "There were a lot of parents on [Fast 7]. Doing Conjuring 2, I get to be the father again." It is possible he is already feeling the presence of a number of parents in the development of Aquaman, which has the added pressure of being part of a shared universe.
Again, this is all conjecture, but it is easy to see how Wan may be having second thoughts as DC's film universe slowly solidifies. Ultimately, he may choose his own comfort as a creator of smaller, but dependable horror films than a cog in the superhero machine.