There's no particular mystery. Navigation in those days was rudimentary - not good enough to fly long distances across the Pacific and try to hit a tiny island at the limit of your fuel.

She missed the island, ran out of fuel, and crashed into the Pacific.

I saw a documentary on her long ago, and she was known to be sloppy in navigation, careless in handling the airplane, arrogant in dealing with her navigator, and would take unnecessary risks with the weather. It's all a recipe for disaster when taking a high risk flight.

It's all too easy to die in an airplane if you don't have a mania for doing it perfectly every time. People also had a hard time believing JFK Jr died because of a perfectly mundane mistake on a routine flight.