In simple terms: It's a Bendis thing, and he does this. It's what I find most repugnant about him, both as a writer and a bad influence on other writers.
The reason the Ultimate Marvel Universe, as such, didn't have a story in terms of why he showed up and why he died really is what it appears to be--Millar wanted as little to do with torpedoing that title as possible. Nothing less and nothing more. It wasn't even a character arc--it was your typical, phoned-in Plot Hammer, thanks to the Bendis/Quesada Regime, with a side order of Politically Correct. Bendis himself hinted at this quite often from day one--that Parker wasn't going to survive his 18th birthday, that he wasn't going to live long enough for Nick Fury to use him.
It's a plot hammer, not a real narrative. Ditto with MM--he's a plot hammer, a reversed male Mary Sue, and not a real character. He's a cipher designed to hit buttons and provoke stereotyping. I've never seen anything more cynical in my life--and that includes _Brand New Day_. Congratulations, Bendis, you've out-sucked Quesada.
You'd think the fans would get fed up with this. You'd think more people would just walk away.
Because the truth is: Peter Parker needed to retire from active heroics ages ago. He did. Spider-Man as character only really works when he's young. There's no question of that: As an adult you really can't be that emo, and that reliant on part-time work. In the real world we call men like that losers and worse. And there's no question that Parker himself was metrosexual if not a deeply closeted bisexual--not something he admits to because his aunt would kill him in _either_ universe, but once the mask goes on, he's flaming and air-humping whatever moves, quite literally.
So what I'm saying is: Ok, yes, this sort of thing needed doing. Absolutely. But there's a right way to do this.
Spider-Man is NOT a damned disposable C-lister. He's not someone you can just _replace_ by plot hammer and plot hammer alone. You really do have to do this the right way--actually have a narrative and write the damned thing and actually make it work.
Not that 21st century Marvel Comics gives a damn, but hey.