Why wasn't Prince Philip Duke of Edinburgh king?

manchestereveningnews.co.uk:

Don't miss a thing by getting the latest from the Manchester Evening News sent direct to your inbox The Duke of Edinburgh – also known as Prince Philip - has died, the Royal Family announced.

He had been an established part of the monarchy for decades. He married the Queen - then Princess Elizabeth - more than 70 years ago.

Around 10 years after they married, Queen Elizabeth officially named him prince, but he never became king. On February 22, 1957 the palace issued a statement saying: “The Queen has been pleased by Letters Patent under the Great Seal of the Realm bearing date 22nd February, 1957, to give and grant unto His Royal Highness the Duke of Edinburgh, K.G., K.T., G.B.E., the style and titular dignity of a Prince of the United