Conspiracy theories are always attractive to so some.
It's much more comforting to think that a rich and influential young woman dies as a result of an elaborate conspiracy, than to accept that she was killed in a speeding car, with a drunk driver and no seatbelts - just like so many others.
If somebody wanted to kill her ( or them ), they would have done much better to do something a few days before, when the two of them were holidaying on a yacht. Boats blow up or sink and are relatively exposed and isolated.
To stage a fake accident in the middle of a big city, in a foreign country, with loads of the world's press in hot pursuit would be total madness and simply couldn't have any reasonable expectation to succeed. There would be far too many unpredictable variables.