I left, as I have stated many, many, MANY times, because I finally hit the end of my rope when it came to Chris and me (and sometimes Chris on his own, and sometimes me on my own) plotting out very specific scenes for very specific purposes in advancing the story, then seeing the finished issue and finding Chris had scripted the scene as something else entirely.
Shooter, at one point, had offered me script approval -- I would be sent a copy of Chris' script to "edit" before it went to the letterer -- but that sounded like too much extra work, especially since I would be expected to do it gratis. So, since I was already set to write the FF, with Bill continuing to pencil, when Bill decided to go with Doug over to MOON KNIGHT, I decided to pencil FF, too.
There was never so much as a single instance of me wanting to do something, Chris wanting to do something else, and Weezie choosing Chris' idea over mine.http://www.byrnerobotics.com/forum/forum_posts.asp?TID=35290&TPN=16
From "Comics Creators On X-Men" (Titan Books - 2006):
Why did you leave X-Men at that point?
John Byrne: I know exactly why. Chris and I were getting further and further apart on who the characters were, and what the characters were about. When Roger Stern was our editor, he worked closely with Chris, to make sure the stories came out as close to what Chris and I plotted as possible. Jim Salicrup, who replaced Roger, sort of leaned in my camp. Then Louise Simonson came in as the editor and she came from the school of thought that says the writer is the important guy. All of a sudden, I had no power. I started to develop what I call my 'Argh' moment -- which is, 'How deep can I delve into this issue before I hit something that makes me go, "Argh!"?' There was one particular issue [X-Men #140] where Colossus is pulling a tree trunk out of the ground with a chain on the splash page, and I went, 'Argh!' right on the splash page because of the way Chris wrote it. I said, 'Okay, this is obviously telling me it's time to go.' There was also a little piece off to one side where Chris and I had argued a great deal about who the characters were and how the characters act, and what they say and what they do and I realised that the Chris version of the characters was what was seeing print. They way Chris wrote it was what was seeing print, regardless of what I thought it was in my head while I was drawing the pages. So if I didn't like what Chris was doing, that meant I didn't like the characters. So when I hit that 'Argh' moment, I basically picked up the phone and called Louise and said, 'I can't do this any more.' I was originally going to stay on for another couple of issues, but ultimately I didn't. The characters deserved better than for me to turn in what was going to be a whole lost less than my best work. So I just left.
From Byrne Robotics in 2010:
>>The way you've described it, JB, you really didn't get tired of the X-Men, just aspects of the collaboration.
Byrne: Yes and no. I certainly grew tired of plotting one thing, and then having Chris script it as something else. That rather goes against the notion of a "collaboration".
But I did get tired of THOSE X-Men. As I've noted elsewhere, I strongly disagreed with Chris' characterizations, and, eventually, I realized that since those were what was seeing print, those were who the characters "really" were. So if I didn't like what Chris was writing, I didn't like the characters!