This is due to the Monarch having the highest title in the kingdom - in this case the Kingdom of Great Britain & Northern Ireland as well being monarch of the Commonwealth Realms.

There is a hierarchy that is followed when referring to nobility, royalty and peerages, known as the Order of precedence. Used primarily in the House of Lord, State Traditions and some 'high-end' restaurants and buildings in London. The Sovereign, regardless of gender is always at the top, followed by their spouse. Since technically a King is higher then a Queen in precedence you can't have a reigning monarch who is a queen higher then their spouse who is a king. Thus, the Queen's spouce - Prince Philip is known as the Prince Consort (consort being an alternate name for spouse of the reigning monarch)

An excellent book for more information is not only the huge number of reports on the BBC website by their royal correspondent but also the large range of books by David Starkey a notable English historian who covers the monarchy indepth.