Here's why:
* "King" refers specifically to a male monarch, the ruler of a kingdom.
* "Royal" is a broader term that describes anything related to a king, queen, or other members of a royal family.
Example:
* King: "The king of England is Charles III."
* Royal: "The royal family attended the ceremony."
While "king" can be considered a royal person, it's not a synonym for the adjective "royal."