Deadpool might be a Marvel comic book, but the film was made through 20th Century Fox, not the Marvel Cinematic Universe.
Despite all this, Deadpool needed Marvel to approve one thing, and that was to change the Negasonic Teenage Warhead's powers.
Writers Rhett Reese and Paul Wernick chatted on the Q & A podcast and explained why they had only two X-Men characters, Colossus and Negasonic Teenage Warhead (who was only really added because of her badass name), and the two didn't even know what her powers were.
For the sake of plotline, the two writers completely changed her powers and therefore had to get Marvel's stamp of approval. Wernick says:
"It was the one thing we needed Marvel’s actual approval on, that they had to reach out for … Tim [Miller, the director] has a relationship with [Marvel President] Kevin Feige and I think he went straight to Kevin because all the lawyers, you know, it gets messy with the lawyers."
Wernick and Reese wanted to change her into "a literal warhead because [they] thought it was funnier." Instead of Deadpool's sidekick being able to see the future, her ability to telekinetically explode was so much more useful. Let's be real, guys. The ability to blow stuff up is a thousand times cooler.
[Source: Cinema Blend]