The long gestation process for Deadpool means the script went through a lot of changes. Early on in the process, Fox actually wanted Cable, Deadpool's grim, gun-toting, pouch-loving buddy to co-star with the Merc with a Mouth -- until Deadpool and Cable creator Rob Liefeld had his say. In an interview with Inverse, Rob Liefeld recalled some early conversations the movie team had:
Half of us believed the movie should involve Cable. I could feel Cable breathing down my neck. I said, "No. Don't put Cable in the first movie." If Cable comes later, great. Deadpool is a vehicle for Ryan to shine. Cable bogs it down, I'm not sure it works as a first film with a general audience. I got in the car and I said, "Did I really just kill the appearance of my other more popular character?"
If true, then yes he did. And thank god for that, because the movie absolutely wouldn't have worked. So much more exposition would have been necessary. Even if Liefeld hadn't said it, we have to believe they never would have actually done Cable and Deadpool first. It would just be too much. And it turned out that being heavily constrained in number of characters really worked for Deadpool.