Why isn't it a queendom then?
Because kings outrank queens, so they decided to use the title of King to emphasize their position as reigning monarchs, rather than as wives of reigning monarchs
There are precedents for female kings: King Christina of Sweden, for example (She is often called Queen Christina, but officially she was a king because that was the inheritable title) and there are several kingdoms in Africa that have female kings if whoever inherits the position happens to be female.