>>123598921>Why the hell didn't they do that?Because both X-Men writers and fandom were in love with the idea of "mutants as a real minority group", so despite Morrison's run literally opening with there suddenly being millions of mutants, and we don't have to explain it, they weren't willing to even entertain the idea of this not being natural (and later writers like Hickman have just tried to pretend the mutant population had always been that big), when the books could instead run on character- and fan- outrage of mutants having had their 'destiny' of inheriting the world stolen from them.
If Decimation had been treated like the reset to an abnormal status quo, the books could have just closed the door on that era and moved on, but instead they made it into something the X-Men spent years fighting to reverse, even though they'd never ever be able to fully reverse it, because it was an editorial edict right from the top. If the goal was to radicalize the characters and the fans, they've succeed beyond their wildest dreams.
Also, Bendis was involved, following on from his work on Avengers Disassembled, which is why instead of using an actual Marvel villain to carry out Decimation, they used the Scarlet Witch as a plot device. Bendis, Brevoort and Quesada didn't believe she had any fans, so they could just use her as a plot device and discard her forever.