Many people are aware that the X-Men film timeline is a jumbled, confusing mess, so it might just be easier for Reese and Wernick to not worry about that too much and just focus on Deadpool’s crazy little world.
That might be difficult to do if Wolverine, Cyclops, or Jean Grey showed up even for just a scene.
It helps set these films apart from the glut of other comic book adaptations, and Deadpool definitely felt fresh and original.
There are also some strong artistic merits to keeping things this way for the time being.