The writer of “Amazing Spider-Man” at the time was J. Michael Straczynski (JMS) — you may know him as the creator of “Babylon 5”.
However, JMS was a big believer in Peter & MJ’s relationship; he even brought them back together after a previous breakup. Moreover, JMS’ run is about Peter growing up. He finds a steady job as a high school science teacher. Aunt May finally discovers her nephew is Spider-Man, so they have an adult conversation about it. “One More Day,” a story about sacrificing your future to cling to the past, is anathema to what JMS previously wrote.
The real mastermind was Joe Quesada, then Marvel’s editor-in-chief (he also personally drew the last two issues of the arc, #544 to #545, and has a co-writer credit on the latter). Quesada disliked Spider-Man being married, feeling it aged the character. In fairness to Quesada, he’s not the only one who feels this way. Roger Stern, who wrote “Amazing Spider-Man” in the 1980s, also feels MJ isn’t the right girl for Peter. Even Gerry Conway — the writer who killed Gwen Stacy and made MJ into Peter’s love interest — has said he thinks Spider-Man should never leave adolescence.
The problem is how “One More Day” splits Peter and Mary Jane up. Aunt May would never support Peter and Mary Jane sacrificing their own happiness for her or them making a deal with the devil. Peter acquiescing to Mephisto is a betrayal of Spider-Man’s ethos, “With great power comes great responsibility.” Worst of all — instead of a simple breakup or divorce, the marriage is wiped from history. Quesada didn’t grow up with Peter and MJ being married, but the current generation of comic readers in 2008 had. So while for him, this was a reversion to the mean, readers saw a betrayal.