From Batman News:

One of the most tried and true methods of landing a major role in a DC movie is to audition for the role of Batman. That’s how Cillian Murphy became Scarecrow and The Dark Knight Trilogy and it’s also how Jason Momoa landed the title role in Aquaman.

Momoa originally auditioned to play Batman for director Zack Snyder in Batman v Superman: Dawn of Justice. Believing he didn’t have much of a chance at getting the role, Momoa gambled with a very different take on the character.