The title of king outranks the title of queen. Therefore it would have been incorrect for Philip to have the title of king. He cannot outrank his wife, Queen Elizabeth II. She was born a princess and became a direct heir to the throne after her father.
If a woman marries a king, she takes the title of Queen Consort. This is a queen by marriage. This is what HRH the Duchess of Cornwall will be when her husband, Prince Charles, becomes king.
Philip could have been given the title Prince Consort, like Prince Albert. However, he remained a prince with a dukedom.