Why is camila a duchess and not a princess?

Camilla is a duchess because she holds the title of Duchess of Cornwall, which was conferred upon her upon her marriage to Charles, Prince of Wales. In the British royal hierarchy, titles such as "duchess" are typically granted to individuals through marriage or royal appointment, while "princess" usually denotes a direct blood relation to the monarch. Although she is the wife of the future king, she does not hold the title of princess by birthright, which is why she is referred to as a duchess.