Director James Mangold had a good reason for not showing The Westchester Incident.
The most heartbreaking revelation in Logan is that Charles Xavier/Professor X, who is now a frail, eldery man suffering from Alzheimer's, inadvertently killed the X-Men, and in his last moment of lucidity, just before he is murdered by Wolverine's young and feral clone X-24, it all comes flooding back to him.
From the audience perspective, we can piece together what happened based on witnessing the powerful and uncontrollable psychic attacks Charles has throughout the film and based on the radio news report we get to overhear, explaining that a horrific incident took place a year ago in Westchester, New York. So, Charles suffered a seizure at the X-Mansion/School for Gifted Youngsters and it caused the powerful mutant to accidentally unleash a psychic blast that paralyzed 600 people and killed 7 — including X-Men.
That's why Wolverine has been hiding Charles in Mexico, housing him in a toppled water tank to dampen the reach of his psychic blasts, and the U.S. government classified his mind as a WMD (Weapons of Mass Destruction).
Mangold wrote The Westchester Incident into the script and has planned to open the film with it, but he changed his mind for several reason. He felt as though the death of the X-Men would've loomed large on the rest of the film. He also wanted to avoid the tired superhero formula of a big opening scene.
“I literally had written an opening which started with that sequence,” Mangold told IGN. “And so it was quite literal, who was dead. But the reason we didn't do it wasn't to spare other films, it was that it redefined the movie. It made the movie about the X-Men, instead of being about Logan and Charles. And irrevocably, when you read the script opening that way, it became about this other tragedy, as opposed to that tragedy being something hovering like a shadow in the background for these characters.”
In the "Old Man Logan" storyline, written by Mark Millar and illustrated by Steve McNiven, something so unspeakable happened that Logan ditched his Wolverine persona and went out west to go live a quiet life on a farm. Through flashbacks, we saw why he stepped away from being a hero. They show him slaughtering a whole slew of supervillains that were attacking the X-Mansion, or so he thought. What really happened was that Mysterio created an illusion that tricked Wolverine into slicing and dicing his X-Men teammates.
In 2029, the mutant population has shrunk significantly and the X-Men have disbanded. Logan, whose power to self-heal is dwindling, has surrendered himself to alcohol and now earns a living as a chauffeur. He takes care of the ailing old Professor X whom he keeps hidden away. One day, a female stranger asks Logan to drive a girl named Laura to the Canadian border. At first he refuses, but the Professor has been waiting for a long time for her to appear. Laura possesses an extraordinary fighting prowess and is in many ways like Wolverine. She is pursued by sinister figures working for a powerful corporation; this is because her DNA contains the secret that connects her to Logan. A relentless pursuit begins … In this third cinematic outing featuring the Marvel comic book character Wolverine we see the superheroes beset by everyday problems. They are ageing, ailing and struggling to survive financially. A decrepit Logan is forced to ask himself if he can or even wants to put his remaining powers to good use. It would appear that in the near-future, the times in which they were able put the world to rights with razor sharp claws and telepathic powers are now over.
Logan stars Hugh Jackman (Logan), Boyd Holbrook (Donald Pierce), Patrick Stewart (Charles Xavier), Dafne Keen (Laura Kinney/X-23), Stephen Merchant (Caliban), Elizabeth Rodriguez (Gabriela), and Richard E. Grant (Dr. Zander Rice).
Logan is coming to Blu-ray and DVD tomorrow!