I think that nerves were still raw emotionally about Diana, who was called the Princess of Wales as her primary title, when Charles and Camilla married, so they chose to use a Duchess designation for her. It's complicated but Camilla absolutely is the princess of Wales because it's a courtesy title held by the wife of the current Prince of Wales.
But she uses the title of HRH the Duchess of Cornwall because HRH Princess of Wales was so strongly associated with Diana. It is the female form of Charles's highest ranking subsidiary title.
Similarly, when Charles ascends to the throne, Camilla could be Queen, Queen Consort, or Duchess, all HRH, and Catherine at that point could use Princess of Wales or maintain Duchess when William takes over Prince of Wales. Which is it's own hot mess because it wouldn't actually be Princess Catherine, her name would actually be HRH Catherine, Princess of Wales, since it's not a title she holds in her own right. Kind of like the difference between a Mrs. before your name, versus an Esquire or MD as a job designator at the end of your name.
At which point theoretically all of Harry and Meghan's children should be "promoted" to HRH prince and princesses.
The speculation has always been that Camilla wouldn't be called Queen, but there's a whole mess if she isn't, because Catherine has to receive the Duchess of Cornwall designation because she'll be married to the current Prince of Wales, and then it's like, well what is Camilla? Dowager Duchess of Cornwall?
I frankly think enough time has passed that Charles can get away with having Camilla designated Queen. But if not, then I'd expect her to at least be Queen Consort.
I mean. Technically any woman married to the monarch is a Queen Consort but like - most of the former Queens in those positions were just known as Queen, but maybe they'll actually add the designator Consort to be used regularly for Camilla?
Also I probably screwed some of this up because it's all very complicated. Like it took me a while to figure out that while Catherine is technically a princess of the United Kingdom (it's her job title, as it were) and would be entitled to be called Princess William right now, she wouldn't be because that's a lower ranking title than Duchess, but that in the future, she may go by Catherine, Princess of Wales, but that she'll never be Princess Catherine, because she gets her titles via marriage.- Mary