Why 'X-Men: Days Of Future Past' Changed A Pivotal Storyline From The Comics

"X-Men: Days of Future Past" is killing it at the box office.

The movie is now the highest-grossing "X-Men" movie to date.

While the film's performing well at theaters, it's also receiving one big complaint from fans.

Based on the popular 1981 comic of the same name, the sequel sends Wolverine (Hugh Jackman) back in time to save the X-Men, and all of humanity, from a group of robot sentinels.

However, in the original comic's storyline it was Kitty Pryde (played by Ellen Page) who was sent back to change the future instead of Wolverine.

Pryde's consciousness is sent back thanks to a telekinetic mutant named Rachel Summers. It helped that Pryde's power is the ability to pass through objects, and in this scenario space and time.

"Days of Future Past" had a huge impact on the comic world - it's been considered one of the most influential storylines ever written - but its biggest impact may have been on the character of Kitty Pryde herself.

The story was one of the character's first big adventures, and helped make Pryde a fan favorite.

In the movie, Pryde is shown to have the telekinetic powers of Summers and sends Wolverine back in time. Many saw this as a nod to Pryde time travelling in the comic.

This is why so many X-Men fans were disappointed with the film's switch (even Jon Stewart questioned the choice).

Why send Wolverine back in time instead of Pryde?

The film's screenwriter Simon Kinberg claims the reasoning was about age.

"The biggest place we diverged from the original books in who gets sent back in time," Kinberg told Empire. "We wanted someone's consciousness sent back in time instead of their bodies, and if you send Ellen Page's consciousness back... she's negative-20 years old, so immediately we realized it wasn't going to the pre-ghost of Ellen Page."

Kinberg says production went through other X-men to send back, but ultimately landed on the series' most popular character, Wolverine (Hugh Jackman).

However, the big switcheroo may be more easily explained by each actor's popularity.

As Kinberg noted, Wolverine is the most recognizable X-Men to audiences. Jackman not only appeared in every single "X-Men" film to date since 2000, including two spin-off movies of his own, but he has also become a huge star in his own right in the past 14 years.

Jackman has gone on to conquer Broadway, host the Academy Awards, and also starred in Oscar-nominated "Les Miserables" along with last year's hit mystery thriller "Prisoners."

Meanwhile, Page appeared in two "X-Men" movies in a supporting role. The actress is best known for roles in "Juno" and "Inception," the latter of which was her last big hit in 2010.

Her movies have made a collective $1.7 billion worldwide (thanks to a big bump from "Inception"). Jackman's movies? $5.1 billion.

The Mary Sue suggested Pryde's lack of popularity was a fault of the writers.

"Falling back on Wolverine yet again is a mixture of kowtowing to audience familiarity and plain old writing laziness touched with sexism."

It's hard to disagree with that argument, but regardless of the reasoning many fans still found the change disappointing.