Sources tell The Hollywood Reporter that Leto was frustrated when Warner Bros. greenlit Todd Phillips' take on the Batman villain because the studio reportedly promised him he could continue his take on the character, which he debuted in Suicide Squad. THR's insiders claim that after the film's 2016 release, Warner Bros. led Leto to believe that he would get his own standalone Joker movie. The studio then turned around and hired Phillips to direct his own version, which as we all know, went on to star Joaquin Phoenix.
Leto was reportedly so miffed, that he complained to his agents at CAA (who also represent Phillips), and told his music manager, Irving Azoff, to contact Warner Bros.' parent company in an effort to get the movie canned.
Leto's disdain for the project may have even ended his time with CAA, which he left last summer. He reportedly felt like his agents should have fought harder to get the movie scrapped and told him about the project sooner. Leto reportedly ended his relationship with Azoff for the same reason. He's now represented by WME, who say that he's working "happily" with Warner Bros. They claim that he wasn't upset about Joker and pointed out that he's currently working on Little Things with the studio.
Leto famously went all in for his portrayal of the DC villain. He spent months getting into character and pranked his castmates with “gifts” like live rat for Margot Robbie, bullets for Will Smith, and a dead pig for the entire cast during rehearsal. Unfortunately, the performance was not well received, and well, here we are.