Short answer: Tradition.

Long answer: Tradition and Politics

The British succession is guided by primarily by the Act of Settlement [1701] (though there have been some small changes over the years].

The spouse of the reigning monarch is not automatically titled when their spouse is crowned. The monarch, with the approval of parliament grants titles to royal spouses.

Traditionally the wife of the King is titled "Queen Consort" and referred to as "Queen". In theory, the husband of a Queen could be granted the title "King Consort" however in practice this has never been done.

The first male consort since the Act of Settlement [1701], Prince George of Denmark & Normandy, Duke of Cumberland, was husband of Anne. He is referred to as Price George, because he was Prince in Denmark. His dukedom was granted by the King who preceded his wife. During her lifetime, senior members of parliament dissuaded Anne from seeking the title of "King Consort" as it was considered it might influence the appearance of her authority for various political reasons (and mostly having to do with the nature of her predecessor's ascension, William of Orange, her brother-in-law).

The next male consort, Prince Albert of Saxe-Coburg Gotha, was the husband of Victoria. He was initially unpopular with the public and the government opposed his ennoblement (the Queen was unable to grant the title without their permission). He declined a peerage title in Britain. In 1857 (20 years into her reign) Victoria formally granted him the title of Prince Consort. So, he was titled a Prince the entirety of his time as the spouse of the Queen, but only as the Prince Consort for the last 4 years of his life.

Finally Prince Phillip, Duke of Edinburgh. Phillip was born a Prince of Greece and Denmark. In order to marry Elizabeth he renounced his titles and the associated claim to those thrones and was raised to the peerage as the Duke of Edinburgh by Elizabeth's father, George VI, on their marriage (1947). He was made "a Prince of the United Kingdom of Great Britain and Northern Ireland" in 1957 (5 years after the Queen ascended to the throne). So, he was titles Prince (of Greece...) until 1947 and Prince (of UK...) after 1957. For the intervening years, he was the Duke of Edinburgh. His getting the Prince of the UK title became incredibly complicated by Commonwealth Countries which all had to "sign-off" on any titles assigned to him.

Generally speaking, there is a hesitancy to assign a male consort any title which includes the word "King" as there is an assumed connotation that the word "King" would denote supremacy over a "Queen" and parliament(s) have wanted it made very clear that the reigning monarch is the one with supremacy over the country/commonwealth.

Further Reading: This wikipedia section actually gives a good overview of some of the controversy surrounding Phillips title & is well cited with contemporary newspaper articles laying out the debate. Scholarly work is rather scant, as it's relatively recent.

Sources:
"Albert: Uncrowned King" Stanley Weintraub. An excellent biography on Albert, which I'm pretty sure had a section discussing the title of Prince Consort which used George for historical context.