No one is/was forced to change their surname. It was a choice then. It is a choice now. People can defy social norms. The royal examples you stated above "might" have chosen to keep their maiden names. I stress on the word might because just because you or me have not heard them being called by their spouses name does not mean that is a fact. It would be a fact if it was changed in their records. Then again when did they start maintaining name changes and records....