During his interview in the newest issue of Total Film magazine, via CBM, screenwriter Simon Kinberg explains why Logan was chosen.
"We made the decision for a lot of reasons, some of them obvious and some of them more nuanced, to make it Wolverine who goes back in time. One reason is that he's the protagonist of the franchise, and probably the most beloved character to a mass audience. Probably the bigger reason is that when we started thinking about the logistical realities of Kitty's consciousness being sent back in time, to her younger self, as opposed to her physical body being sent back ... it was impossible.
"Obviously in the book it's Kitty ... but you're talking about an actress [Ellen Page] who, in the age of Michael Fassbender [a young Magneto] and James McAvoy [a young Professor X], would have been negative 20 years old. So we started thinking again, and the first reflex response to that was a character who doesn't age. Wolverine is the only character who would looks the same in 1973 as he does in the future."
Jackman is the franchises biggest star since the first X-Men film hit the big screen back in 2000. And after The Wolverine became the second highest grossing X-Men film in 20th Century Fox's X-franchise, it made sense to continue using Wolverine as the lead in X-Men Days of Future Past.