Originally Posted by wobbly
Because it's too a major departure from the story, and one that avoids showing us the fates of those surviving members who stayed behind.
They did, making the success of Kitty's mission all the more important. There was no happy ending for that future despite their best efforts, but they at least tried to fight for one, rather than all of them jumping ship in an escape to the past.
How do they even know the time travel will work? If it all goes tits up on them, they all get wiped out and no-one is left in the future to fight the Sentinels. You don't put all your eggs in one basket with something like time travel.
Then there's the event, the method of time travel used etc.
Kitty was chosen as she was the only one among them who had not yet been trained against telepathic attacks. Kitty could still be chosen as her power could make her the only one who could survive the process.
With an existing team in the past, you don't need all the OT guys going back too to create the required change.
So what if its a major departure from the story. This is not gonna be a 100% adaptation of the comic-book storyline.
All of the X-Men members were present (except for the ones who were injured, died and went to get a cure) when they did their mission on Liberty Island, Alkali Lake and Alcatraz. Having all of the X-Men to time-travel to the past is not impossible to imagine.