Good point, Frank. I'm just speaking on a wild guess, but not only looking like a Regency if the Duchess of Cambridge attended, we'll never know, but perhaps Catherine herself felt her presence would take the attention away from Charles. Go back to the Opening of Parliament when Diana showed up with the "new hairdo" and overshadowed the Queen and entire Ceremony. This just my speculation, it may have been decided that Catherine would not attend or planned in such a way.