Maybe because in a reboot, that's all anybody ever wants is colorful comic costumes, cosmic goddess Phoenix, and basically an all around flashier movie with no regard to the human element of X-Men.
I never once said the fantasy didn't belong in an X-Men movie. It's a comic book movie, of course it does.
But the moment it begins to neglect the human element is the moment I check out. And when that's all anybody is clamoring for with a hypothetical reboot... you wonder why I associate reboot with that?
When I say "grounded", I mean that first and foremost, the movies are about the character, and the foundation of the films is a society where these mutants are oppressed and ostracized and the film is built upon that, where the displays of powers, the costumes, and the fantasy elements such as Sentinels and time travel simply enhance that.
But people have been whining since 2000 about how the X-Men movies adapted the source material, and want something that is the opposite, where the film is built around displays of power, colorful costumes, and huge sci fi set pieces, and the humanity of the film is merely an aside to the comic book action pieces.
And if the movies are going to remain serious and based on the human foundation, then why even do a reboot when the last entire series was based around that? It wouldn't be anything new.
There's a reason why I am adamantly against an X-Men reboot - there is literally no purpose that a reboot would serve outside of raking in money for the studio.
And if the recent returns of X-Men: First Class and The Wolverine are anything to go by... the studio won't be raking in tons of money with this franchise anyways. It's definitely on it's last legs and should be coming to a dignified end, not dragged through the mud with pointless reboots and needless spinoffs.
At least by continuing the current series, we can expand into new stories that haven't been told yet, instead of rehashing the same crap over again with a reboot, this time just more colorful