I think the reason for the omission is just a standard set by the royal household. I read somewhere else and a long time ago, that the Court Circular will never refer to any person's foreign nobiliary title unless the monarch decides to make an exception for some special occasion or event. One example that was given was the state dinner given for Pres. John Kennedy and Mrs. Kennedy, when Mrs. Kennedy's sister Lee, Princess Radziwill and her (then) husband Prince Radziwill were guests, and were on that occasion referred to as Prince and Princess Radziwill.