By committing to single, standalone films that don’t tie into larger universes, DC can bring in filmmakers and actors who want to focus on one project at a time, which pretty much guarantees that everyone involved will bring his or her A-game to the proceedings.
A guy like Joaquin Phoenix arguably does not want to find himself locked in for a multi-picture deal that could eat up to a full year of his life, which is why such a one-off movie could be exactly what the genre needs to keep attracting talent of that caliber.
One of the most intriguing elements of the Joker solo movie is the fact that Joaquin Phoenix appears to be circling the project as the frontrunner to portray Mr. J. The actor has flirted with comic book movies before (particularly Doctor Strange), but he also has not shied away from elements of the genre that he’s not interested in pursuing.