Because he is ill?
Because Wills is more popular?
Because he is planning to hand over the throne soon?
AFAIK, King Charles is not in good health, thus he sent his son, the heir to the throne, in his stead.
Just as he himself had stood in several times on state occasions - during the last years of Queen Elizabeth II, his aged mother, after she had fallen into feeble health.
He is struggling with cancer, and I am praying for his recovery.✝️