Reading the script for Spider-Man: Turn Off the Dark (the first version) and comparing it to the Sam Raimi films (which it takes inspiration from), and remembering Glen Berger's comment in Song of Spider-Man about how Julie Taymor did not find Norman's backstory (a businessman who was just trying to secure a military contract) compelling, which is why he's dead by the end of Act One.
But like... the main interpersonal conflict of the Raimi films isn't really Peter and Norman. It's between Peter and Harry, Norman's son (who is suspiciously absent from the musical). Norman's Goblin serves as Peter's first test as Spider-Man, but it's Norman's death that catapults Harry into becoming Peter's enemy. That would have been great to explore in a musical.
But that wouldn't have left any room for Arachne, and Taymor was dead set on including her. And we all know how that turned out.
