Why was Britain's Queen Elizabeth The Queen Mother . . .?

Queen Elizabeth, the Queen Mother, passed away some 9 years ago. She was the MOTHER of the Queen Regnant, and also a QUEEN, so..."Queen Mother". Mary didn't CHOOSE to use the title Queen Mother. Usually, the widow of a king is known as the "Dowager Queen", but, because she was still so young, and a Dowager carries an implicit connotation of being very old, she CHOSE to be known as the "Queen Mother". Queen Mary had no one to be confused with. See, there wasn't anyone else by the name of "Queen Mary" at the same time, so, there was no need to USE any additional title on a regular basis. The Queen Mum and the Queen Regnant, both being "Queen Elizabeth", it became immediately necessary to have an additional moniker to differentiate between the two women.

Queen Mother is a title or position reserved for a widowed queen consort (a queen dowager) whose son or daughter from that marriage is the reigning monarch.[1] The term has been used in English since at least 1577.

Elizabeth CHOSE that title because she did not like the name "dowager."

The alternative would be to call her "The Dowager Queen Elizabeth", and her daughter "Queen Elizabeth II". But no one wants to be called "dowager" today, so the relatively minor title of "Queen Mother" was used instead. Given the affection that most people felt for her, the title was frequently shortened to "Queen Mum".

Elizabeth Bowes-Lyon (4 August 1900 – 30 March 2002) was the Queen consort of King George VI from 1936 until her husband's death in 1952, after which she was known as Queen Elizabeth The Queen Mother, to avoid confusion with her daughter, Queen Elizabeth II. She was the last Queen consort of Ireland and Empress consort of India.