One of the best recurring jokes in the original Deadpool movie is the often-commented absence of actual X-Men in this X-Men property.
Sure enough, when it came time for the much-anticipated sequel, which was accompanied with a bigger budget and a higher profile release, there was the strong likelihood of at least a few well-known X-Men making an appearance.
There are only two other characters from the series who made appearances in that film, and Deadpool is often disgruntled by their non-appearances in his film.
But their cameos needed to fit the style of the film, and the filmmakers wanted to honor the winking satire that is crucial to the film’s irreverent success.