>>94899935When the X-Men ongoing got canceled with #66, later writers/artists randomly decided to say "Fuck It" and put Scott, Angel, and Jean back into their old yellow/black costumes since they were effectively D-Listers in the eyes of Marvel at this point.
Of the three, only Angel had a reason given (Magneto created the Neal Adams designed costume Angel wore, which under Englehart, turned out to have a bunch of secret circuitry in it that led Magneto leech his life force and control him.
Scott and Jean were back into their old costumes IIRC in Avengers #110-113, which was written by Englehart and drawn by IIRC Don Heck (an old school artist who might have been the one to unilaterally bring back the old costumes). It was also the story where Angel's Neal Adams costume got revealed to be booby trapped.
In X-Men The Hidden Years, John Byrne started a plotline where he intended to reveal WHY the Scott/Jean at least started wearing their old costumes again and had Jean in her first appearance costume, out of the blue, in the last couple of issues of THY.
Byrne never got to give the official reason, but it's heavily implied for Jean at least, that Xavier used his powers on Jean (and ultimately Scott and Warren) to force them back into their old rookie costumes, because he was getting pissy at how independent the team was getting and wanted them back under his thumb, complete with them being stripped of their unique costumes.
