Fans of superhero cinema may recall the rather odd existence of two versions of the character Quicksilver, one played by Aaron Taylor-Johnson from Avengers: Age of Ultron, and another played by Evan Peters in the X-Men series. How did this happen? It has everything to do with which studios hold the rights to which characters, and Cable, like Quicksilver, is on somewhat shaky ground.
Spider-Man, the Fantastic Four, and the X-Men are all Marvel Comics creations, but due to how the movie business functions, Marvel Studios doesn't have the rights to make movies about all those characters. The Fantastic Four and the X-Men film right s all live at 20th Century Fox, and if you've ever read an X-Men comic, you know it's a team that tends to sprawl. In the comics, Quicksilver and Cable are mutants, but they don't solely appear in X-Men properties. They show up in other places, and they function on their own. Which makes things tricky from a rights perspective. In the Quicksilver example, Age of Ultron's iteration simply wasn't a mutant, and neither was the Scarlet Witch. While both characters appeared in Avengers comic books for years, their origins as mutants were erased and altered to better fit the needs of the MCU…and to avoid rights issues with the X-Men movie franchise. Meanwhile, Fox could use him as a mutant (and the implied son of Magneto) in X-Men: Days of Future Past.
Cable has the same problem. While the rights to the character would seem to be with Fox based on his mutant origins, Marvel has reportedly done tentative work in getting a movie of his developed as though the rights are theirs. Partnerships between studios are not unheard of, of course—the upcoming Spider-Man: Homecoming is a Marvel/Sony co-production—but this rights wrinkle does add one more complication to the stack of reasons why Cable is a hard prospect for a solo feature. But perhaps there is another way.