Originally Posted by
Kaitou D. Kid
It's mostly because no one ever permanently replaced Norman's role as archenemy (in spite of guys like Harry and Kingsley having cult followings), while Gwen's role as the main love interest was completely successfully replaced (and eventually surpassed) by MJ.
After that, the only way Gwen could have came back was if you brought her back in a new role like with Red Hood and Winter Soldier. Like, if Felicia was never introduced, maybe you could've brought Gwen back as Black Cat and have her be the love interest that's completely disconnected from Peter's life. I guess that could have worked? It would even fit with the "one that got away" feelings that Black Cat eventually started having for Peter.
But they didn't do that. Plus Spider-Gwen now exists. She already is "Gwen in a new role". Which just further reduces any need to bring back the original Gwen.