According to Marvel: The Untold Story...

First, Louise Simonson (New Mutants) quit due to conflict with Bob Harras and Rob Liefeld.

Editor Bob Harras was not liking the direction Claremont was taking it, and he was agreeing with Jim Lee and Whilce Portacio's plotting. Harras gave plotting to Portacio and Lee; Claremont was reduced to writing the dialogue for their plotting and art.

Jim Lee was sending the artwork to Claremont so late that occasionally Claremont would have to write the script overnight.

"As they approached the big launch of X-Men, Claremont said, the battle with Harras became 'an outright knock-down drag-out fight.' Harras wanted to bring Professor X back into the stories; Claremont wanted to kill Wolverine and complete Magneto's transformation from villain to hero." (p. 328)

"Claremont and Harras began communicating exclusively via fax machine so that there would be a paper trail of the increasingly tense exchanges."

"After Claremont's wife reminded him that they had a mortgage to pay, he negotiated to write the first three issues of the new X-Men; this would be, in effect, his severance pay."

Claremont said, "What you have is a corporate disagreement between an employee and his supervisor. And in that light, the course of action becomes as clear as it is inevitable: the corporation instinctively supports its supervisors."
The great shame is that the sales, Harras will say, prove him right. The dramatic decline from the consistency of Claremont's run and prominence of editorial interference to the detriment of story quality proves him wrong in the long term.

But I also think another shame was that the X-Men animated series hit exactly when it did. Had it come a year earlier, we'd likely have seen the Cockrum and Byrne designs; something more classic. But that cartoon lionizes Jim Lee as well as Rob Liefeld and due to that, we have an entire generation of fans that think that era as well as characters like Stryfe and Onslaught are the height of the franchise. The X-Men hit the height of it's popularity when it's quality began a steep decline.

Not to mention this sounds like it would have been better than X-Tinction Agenda, Fatal Attractions, Age of Apocalypse, etc: https://nerdsonearth.com/2017/05/chr...e-mutant-wars/