Why was Diana a princess but Kate is not?

Lady Diana Spencer married the Prince of Wales in 1981, and as a result of their marriage, she was given the title of Princess of Wales.

However, the title of ‘Princess Diana,’ as she was known to the public, was not correct.  Diana was no longer called Her Royal Highness after her divorce from Prince Charles in 1996.

Alternatively, until her death the following year, she was known as Diana, Princess of Wales.

When Kate Middleton married Prince William in 2011, she assumed the equivalent of his new Duke of Cambridge title, which the Queen had bestowed on him.

Kate was entitled to the title of Duchess of Cambridge because she was William’s wife. Kate is also called Her Royal Highness (HRH), but she has never been referred to as Princess Catherine or Princess Kate in public.

If William hadn’t been conferred a Dukedom when he married Kate in 2011, Kate might have been referred to as Princess William instead.