I've often heard it discussed whether it was because MM is an American that she made so many errors in her short time as a Royal and never fitted in.
But when you look at some of the current European monarchies ~
The Queen of Denmark is Australian
The Queen of the Netherlands is Argentinian
The Queen of Sweden is German/Brazilian
The Princess of Monaco is South African
In addition to learning the intricacies of their new country and the various aspects of royal life, they've all had to learn a new language but all of them have done extremely well in their royal roles. Especially the Queen of Sweden who became a Queen on her wedding day as her husband was only 27 when he became King
So it's obviously got to do with MM's personality and, I think, her inability to learn, take advice etc. I've read several different versions of the help that was offered to MM by BP and they all seem extremely sensible for a young woman from a different background. Her refusal on the grounds that Harry would help her was extremely naive as (a) he wasn't experienced enough in Royal duties at that stage and (b) he wouldn't have known or understood the subtleties of dress codes for royal ladies. His last minute tuition in teaching her about curtsying to HMTQ is a prime example of his failure to prepare her and her poor attitude to the correct etiquette
I really feel the whole romance ~ engagement ~ marriage was too hurried in the circumstances and PW was right to tell his brother to take his time over it. Shame PH got in a huff and ignored his brothers advice