James Gunn has a pretty good answer as to why Joker doesn't need to be included in The Suicide Squad. Jared Leto's Clown Prince of Crime was featured in 2016's Suicide Squad, albeit briefly. Leto was all over the promotional material for the movie and many DC fans expected him to have a large part in the project. Even Leto himself thought he was going to have a bigger role. That was not in the cards and it looks like Gunn may be keeping him out of The Suicide Squad altogether, though he doesn't outright confirm that.
James Gunn took part in an impromptu Q&A Instagram story session yesterday and spoke about a variety of different subjects with his fans. There were a lot of questions about the Marvel Cinematic Universe and Guardians of the Galaxy Vol. 3, but there were also a lot having to do with The Suicide Squad. Gunn had this to say when asked about the lack of Joker in the movie.
"No one but me and a few others know all the characters in the movie, but if the Joker isn't in the film, I don't think it would be strange as he isn't a part of the Suicide Squad in the comics."
Obviously, James Gunn could actually have the Joker show up in The Suicide Squad for a cameo, but even that seems doubtful at this time. The cast is incredibly big and there just doesn't look like there's any room for the Clown Prince of Crime to fit in. With that being said, Todd Phillips' Joker is currently the number one movie in the country, and it has been for over two weeks now.
Joaquin Phoenix introduced a brand-new take on the iconic villain in Joker and all eyes are currently on him at the moment. Jared Leto was apparently caught off guard and angered by the movie, which takes place outside of the DCEU, so we might not see the actor coming back to reprise his role, even though there are quite a few projects on the horizon that could feature him. Birds of Prey is one such project and it apparently does have a Joker cameo, but not with Leto in the makeup.
Margot Robbie's Harley Quinn, Jai Courtney's Captain Boomerang, Joel Kinnaman's Rick Flag, and Viola Davis' Amanda Waller are all back for The Suicide Squad. John Cena, Taika Waititi, Idris Elba, Nathan Fillion, Peter Capaldi, Sean Gunn, Pete Davidson, Flula Borg, Daniela Melchior, David Dastmalchian, Gunn, Jennifer Holland, Mayling Ng, Michael Rooker, Daniela Melchior, and many more are set to make their Suicide Squad debuts. James Gunn is keeping everything under wraps in terms of who is playing which character in the movie, which he is very good at from his experience working for Marvel Studios. Gunn will keep us posted as the production continues. The James Gunn Instagram story Q&A is still up as of this writing.