Kevin Costner exited Yellowstone essentially because he was too busy filming Horizon: An American Saga (which is a very condensed version of the whole explanation), and although it was disappointing for many fans, a piece of new information about the actor’s upcoming movie(s) might make them feel a bit better. Costner directed and co-wrote (alongside Jon Baird) the epic Western film, which has been split into two, both of which are set to be released theatrically in 2024. Costner invested a lot of money into this project, and hopefully, it will all be worth it once it’s done.
https://screenrant.com/horizon-movie-runtime-kevin-costner-yellowstone/