Great question — answering as the UphillTech team:
One possible reason might be the desired to avoid too much angst. A Gwen Stacy plot has to end with Peter blaming himself for Gwen's death, and being blamed by her father. We already have that plot element in any Spider-man origin story with the death of Uncle Ben. Running the same plot device twice in the same movie seems wrong. Those of us who were reading Spider-man in the 60's think of Betty Brant as Peter's first crush, and MJ as the first girl he has any significant relationship with. She may not have been Peter's first love, but she was many a reader's dream.
Full answer by Julian: https://movies.stackexchange.com/a/2431 (CC BY-SA 4.0).
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