James Wan chose Aquaman over The Flash because “the bar is a lot lower”
Upon being presented the choice of directing a movie based on either The Flash—a relatable guy who gleefully unleashes his superspeed powers to fight crime—or Aquaman—a guy who spends his time polishing a trident and talking to fish—most people would presumably choose the former. However, director James Wan (Furious 7, The Conjuring) chose the latter, basically because fewer people will admonish him if he screws it up.