Why king and queen rather than king and kingess?

Queen has its origins in a pre-English word meaning simply ‘wife’. Beyond that, we need to look for anthropological and social, rather than linguistic, reasons why a king’s wife should not have had a more distinctive description.

Lord comes from Old English hláford, meaning ‘keeper of the bread’. Since this was presumably a role denied women, the need for a feminine form didn’t arise.

Queen is a very old word. It has been in English since before 900. There was never a kingess. The term queen is related to Greek gynḗ (woman) and probably to Latin re-gin-a meaning queen.

This is pure conjecture, but I think it's because there is only one. In the peerages of British nobility, there could be multiples of all other nobles, but there could only be one King and Queen. It stands to reason that her name would be unique and individual.

"Queen" comes from "Quenna" (the wife of "Quenno") in protoceltic language (perhaps 500 BC. Quenno is the "head" (Head/Leader of Warriors/Soldiers). The Quenno is not the King! King is a protogermanic idiom (Kuningaz). The wife of the "Kuning(az) is "Kuningin". The celtic King is "Rigs" (latin = rex). Because a "King" is everytime the "Regent", but not his wife.

It has to do with the origins of the words.

-ess words

All the words that end in -ess came to English with the Norman Invasion from French and Latin. In fact, the suffix -ess is itself derived from French -esse. So words ending in -ess are of French origin. Almost all of them came to be used from Middle English (ME) onwards:

• duchess: from ME duchesse
• goddess: from ME goddesse
• princess: from ME princesse etc.

King and queen on the other hand come from Anglo-Saxon.

King and queen

King and queen are both native English words--Anglo-Saxon words:

• king comes from Old English cyning (alt. cyng)
• queen comes from Old English cwēn

cwēn originally meant a wife, specifically that of a king or another important man. Etymonline says that the original sense (i.e. wife) has been specialized by Old English to wife of a king.

Larry Trask suggests that the current meaning of 'queen' is 'improved' because of 'Melioration' (Semantic change). Melioration is an improvement in meaning. He says that queen formerly just meant 'woman', but today it means queen (the female monarch of a kingdom). [Trask's Historical Linguistcs]

In summary, all the -ess words are French/Latinate words while king and queen are native words and don't take these endings.