why did he get kicked from the disney cinematic universe?
Before Disney was its own motion picture studio, they were an animation studio that produced projects for other studios such as RKO Pictures and Universal. Oswald was made by Disney for Universal and when Disney was removed from the Oswald cartoons by Universal, they made Mickey.
Years later, Disney would regain the rights to Oswald in a trade with Universal that saw Oswald return to Disney in exchange for an ESPN Sportscaster going over to NBC Sports. Disney eventually included him in the Epic Mickey video game series.
There is now an Oswald series in development for Disney+.