I think the idolization stems from her "persona" and the fact she died well over 50 years ago under somewhat mysterious circumstances. Throw in the idea she was relatively young when died (36), and it perpetuates the myth in a way.

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Think of James Dean. Similar to Monroe (both died young and are still idolized to this day). While Dean's death was far more obvious, the fact he died young adds a kind of mystique and allure that wouldn't be there had he died at a much older age. I feel it's a very similar concept with Monroe