(long essay time... you shouldn't have gotten me started)
Because these are characters that are very beloved to us. I myself, have been a huge X-Men fan since a child. I grew up with these characters, my first exposure to them being the animated cartoon on Fox in the early to mid 1990's. So, I've been a fan of the X-Men for somewhere around 15 years now. And there are some of us here who have been around much longer than that, and have been fans of the X-Men for the past 40+ years since the comic book has been in existence.
When I was younger, watching the cartoons, I knew I loved the X-Men, and as I've gotten older, I realized why. Sure, superheroes and comic books are fun, but to be honest, I'm not a huge comic book fan. However, X-Men strike me as something different, because they are fun, fantastical characters, with a true, deeper meaning behind them. There is depth.
The reason why some of us are upset, concerned, scared, or however you want to put it, is because it appears that in this movie, these characters won't be brought to life. They are just going to be shells of what they really are, based off of a name, a power, and a general appearance.
Do you know why the Lord of the Rings movies are so beloved? Because not only were they amazing movies, but they showed the utmost respect to the source material. My mother is a HUGE LOTR fan from the novels, and she is absolutley thrilled with the movies. Sure, it had it's changes, as any adaptation from one form of medium to another will have, but it remained true to Tolkienn's work.
In my opinion, and the opinion of many others, Bryan Singer's take on X-Men and X2 was a very accurate representation of these characters. Changes made, yes, because it's bad enough to adapt a novel, with a self-contained storyline into a 2 hour movie (or 3 three hour movies), in the case of LOTR), but to do that with 40 years worth of material, much of which has contradicted itself over that time, it's very hard to not have deviations.
But despite all of the deviations, overall, Bryan Singer was accurate to the depth of the source material, the world, and it's characters. To me, watching Bryan Singer's movies, I truly felt like I was watching the X-Men. Some people will complain about Wolverine's screen time, or Cyclops' lack thereof, but I believe it's nit picking, and not accurate to what was truly displayed.
With this movie, however, it seems to be the opposite. This movie is adapting one of the most admired story arcs in the history of the source material, and yet, this film seems to be taking the biggest creative "liberties", if you actually want to give it that much justice.
We are going to have arguably the most important character in the X-Men world killed off, something that never happens in the source material. And not only that, but it is happening in a story arc that he is supposed to be a major player in.
We're going to have Wolverine take command as a leader of the X-Men. Never has Wolverine been a leader, he is not a leader type. He is definatley not the poster boy for Xavier's ideaology like Cyclops is.
I won't complain about Storm stepping up into her role, because despite the fact that her role should have gone to Cyclops, her role IS accurate to what Storm would face should Cyclops and Xavier ever pass on.
These aren't minor changes to help transition the material from comic book page to big screen film. These aren't changes like altering Lady Deathstrike into a mutant who has the exact same powers as Wolverine, who also had the adamantium procedure done. This isn't a change like changing William Stryker from a reverend, or priest, or whatever he was, to a Colonel (which, I believe even his comic backstory had him as a retired colonel before becoming a priest).
No, this is taking the leader of the X-Men, arguably the most important character in X-Men history (even if he isn't the most popular, or famous, or recognizable), and killing him off early in the story to play a non-factor in a story arc that he has major involvement in. This is taking a very well known character in Wolverine, and turning him into a house trained leader, not the rebel who does things his own way.
You are screwing with 2 major characters, quite possibly the 2 biggest characters EVER in the X-Men, and turning one into something that he's not, and killing off the other when there's no source material precedent for it.
It's taking quite possibly the most beloved story arc of all time within the source material, and totally changing it up, focusing on a lust as the savior, instead of the true love that should be the savior.
Imagine having Aragorn killed off in Lord of the Rings, and Frodo becomes King of Gondor. How well do you think that would sit with fans of the source material? And do you think they'd accept some kind of "Well this is the movie, not the book, and they had to work within the parameters to make the movie work" excuse?
That is exactly what this is.
Again, I'm not against change. I embrace the change from comic book page to big screen film strip, as I understand that not everything can remain 100% faithful. BUT, despite the fact that this isn't the comic, you are still making an X-Men movie. If you are going to make a film that's not loyal to the source material, then you shouldn't call it X-Men. You shouldn't call these characters Cyclops, Wolverine, and Xavier. Changes need to be made, yes, but there needs to be a line drawn somewhere. And killing off Cyclops, and turning Wolverine into a leader and Jean Grey's savior, crosses it, and crosses it big time.