it was clearly stated in the X-Office, I can't remember right now if it was in the Morrison Manifesto in the New X-Men HC, or on an interview with Morrison, as well as Claremont's X-Fan board somewhere I think, that they weren't allowed to go back into the past and add details to events that never happened, so as not to confuse readers.
I definitely remember that it was a line of Claremont's that editorial didn't want confusing-to-new-readers things even mentioned, such as Bishop being from a future possible timeline. An abosulte "no you can't, ever!" is a bit absurd, but I can understand the reasoning behind it as it often does create confusion. Claremont rose to the challenge well, giving Bishop lines like "I was a cop where I came from." New readers can accept that, old readers already know where he came from. And, this being Claremont, further explanation might have been too long winded.

Claremont also serves on the confusion angle here, also with X-Treme. My case in point is Magma.
She comes from "Nova Roma" a time-lost remaining colony of Ancient Rome, where they still had gladiatorial fights (a fight plot I personally detest as ultimate cliche.)
Later writers also found that lame. Instead of ignoring it, they filled in backstory that all the citizens of Nova Roma were kidnap victims brainwashed into thinking they were who they were. Magma, then known as Amara Aquilla, was revealed to be an Englishwoman named Alison Crestmere. The character was used well when she was used, and subsequent writers even followed through with Claremont's old future-hints that Magma was one New Mutant "destined" to go to the dark side.
Claremont didn't like the erasure of Nova Roma, so recently undid the kidnap victime plot. He literally tore Mogma from the recent New Mutants book and began reusing her without explanation as Amara rather than Alison. In last month's books, he finally explained this, but I'm still sitting here with question marks dancing atop the old noggin while I cringe at the thought of new arena gladiator stories yet to come.
X-Force #87-90 is a good read of the "interim Magma" and worth hunting down.

I believe Byrne's X-Men Hidden Years was axed due to the no-confusion policy, a real shame as it was a really good book, worth picking up as back issues. Yet another example of how some books are axed for reasons other than low sales!

Unfortunately, I don't think Marvel has the investment of resources and editorial personnel it once did to keep these things straight. They hear too much of every gaff, so would rather reduce the chances of making the gaffs by limiting the writers.

Marvel's refusal to use footnotes is a far worse policy. Scott Lobdell is using the Plodex now, and should be allowed a footnote or letter page text box that would read, "The Plodex originally appeared in AF Vol 1 "2-4, 14 - 16, Marvel Team-up Annual 7, now available in the trade paperback, "Alpha Flight, Plodex Eat Spandex." The trade could also include the AF/Avengers crossover where Marrina and Namor married. Thus the history is preserved, new readers aren't confused, and Marvel has some trade sales from newer readers looking for some history.