I would have thought it was obvious - she's half BLACK - it's racism pure and simple - the UK press hate her for being half-caste!
The British royals have a habit of making any member of their family as miserable as possible if they marry for love or even consider marrying for love (See Princess Margaret, Prince Charles, Edward VII for confirmation of this!). The British royal family marry out of DUTY, not love!
Sadly, Diana, Princess of Wales didn't seem to be aware of this!