Because Prince Charles was - and still is - the Prince of Wales when she married him.

Her name was never Princess Diana. He correct title was Diana, Princess of Wales.

If you mean Diana, the Princess of Wales. She is dead.

• After Princess Diana's death the 'Diana Memorial Fund' was granted intellectual property and sued the Franklin Mint (they had much of her jewelry, dolls, etc.) and because they refused to produce Princess Diana's merchandise the Memorial Fund sued them, but lost the case held in California and the case was settled in 2004 with the fund agreeing to out-of-court settlement which was donated to mutually agreed charitable causes. Today, after this lawsuit, two California companies continue to sell Diana memorabilia without the need for any permission from Diana's estate: the Franklin Mint and Princess Ring LLC.

Diana became HRH Princess of Wales when she married Prince Charles on July 29th, 1981. After the divorce in 1996 she had to give up the HRH (Her Royal Highness) and became Diana, Princess of Wales. She was never officially called Princess Diana.

Diana, Princess of Wales