Continuity mistake: During the interview for the commission, Arthur's psychiatrist asks him about his mother's voice. He replies, "What does that mean?" cigarette in hand, barely above the table. In the next shot, the cigarette is in his mouth. She asks a few seconds later about Joker, "Would he like to talk to me?" Joaquin Phoenix is smoking his cigarette at a different angle between shots.
Continuity mistake: When the barber, towards the beginning of the movie, asks Arthur if he's got a joke for them, Joaquin Phoenix's face is cleaner from soap compared to the previous and following part of the scene - when it's thicker on the neck and goes all the way up on the cheekbone.
Continuity mistake: During the cartoon short that opens the movie, Joker, on his way to the stage, breaks the arm of a man in the corridor. In the background, slightly ahead of him in his path, is a poster for Charlie Chaplin's Modern Times. The camera angle changes as Shadow Joker turns around; he is supposed to be in the same spot, but the poster is way behind him; Joker is now standing next to the Pal Joey poster.
Answer: For the most part, it was likely down to the fact that the first film was a massive smash hit, grossing $1 billion on a budget of less than $70 million. There was simply too much financial incentive for the studio to not greenlight a sequel. The director also reportedly hated that the character, as portrayed in the first film, had become something of an "incel" poster boy and was idolised by part of the audience. So he wanted to make a sequel that tore down this notion by reaffirming that Arthur was just an unhinged, mentally ill weirdo.
TedStixon