Because Elizabeth was heir to the throne and Prince Philip married into the family.

Marrying a Queen does not grant you a title as King. I refer you to http://en.wikipedia.org... http://en.wikipedia.org... and Q: Why is Prince Philip not King Philip? A: In terms of precedence a King is higher than a Queen, which means that in every case where there is a King and a Queen, the King would outrank the Queen. When it is a Queen who holds the throne in her own right (a Queen regnant, like the present Queen Elizabeth II), her consort has been referred to as a Price to make the point that it is she that takes precedence. Therefore, the Queen Mother was Her Majesty Queen Elizabeth (Queen to King George VI), but Queen Victoria's husband was Prince Albert and Queen Elizabeth II's husband is Prince Philip. The only example where both a King and a Queen have held the throne is with King William and Queen Mary at the end of the 17th century. This was because they held the throne jointly. http://www.etoile.co.uk/ In May 1954 the Prime Minister, Sir Winston Churchill, received a written suggestion from the Queen that her husband be granted the title "Prince of the Commonwealth", or some other suitable augmentation of his style. Churchill preferred the title "Prince Consort" and the Foreign Secretary preferred "Prince of the Realm". While the Commonwealth prime ministers were assembled in London, against his better judgment but at the Queen's behest, Churchill informally solicited their opinions. Canada's leader was the only one to express "misgivings". Meanwhile, the Duke insisted to the Queen that he objected to any enhancement of his title, and she instructed Churchill to drop the matter. [4] In February 1955, South Africa belatedly made known that it, too, would object to the "Prince of the Commonwealth" title. When told, the Queen continued to express the wish that her husband's position be raised, but rejected the Cabinet's recommendations to confer upon him either the title "Prince Consort" or "Prince Royal". By March 1955 the Cabinet was recommending that Philip's new title be simply "His Royal Highness the Prince". But the Queen was advised that if she still preferred "Prince of the Commonwealth" her personal secretary could write to the Commonwealth's governors general directly for their response, but warned her that if their consent was not unanimous the proposal could not go forward. The matter appears to have been left there until the publication on February 8 1957 of an article by P. Wykeham-Bourne in the Evening Standard titled "Well, is it correct to say Prince Philip?" A few days later Prime Minister Harold Macmillan and his Cabinet reversed the advice of the previous ministers, formally recommending that the Queen reject "The Prince" in favour of "Prince of the United Kingdom of Great Britain and Northern Ireland and Her other Realms and Territories", only to change this advice, after she consented, to delete even the vague reference to the Commonwealth countries. Letters Patent were issued, and according to the announcement in the London Gazette, the Queen's husband officially became His Royal Highness The Prince Philip, Duke of Edinburgh. She inserted the capitalized definite article, a usage normally restricted to the children of monarchs.