Why wasn't he a king?

He was commonly known in the UK as the "King of Hearts" and "The People's King" so in a sense he was.

Because he wasn't in line to the throne

He could only become a Prince and he didn't receive that title until 1957. It was the same when Albert married Victoria to become Prince Albert. I don't think there's ever been a title of King Consort when the Queen is the hereditary monarch. Philip had to agree that any children couldn't bear his name when he married Elizabeth. And he had to agree to give up smoking too.

There are certain laws regarding ruling monarchs and their consorts. Marrying the ruler of a country does not automatically make you a king or a queen, nor would you have the power to take over and steal the throne from your spouse. Too many power-mad idiots in history did that in the past, leading to all the legal restrictions that have been put in place since.

Apparently Prince Philip's family had ties to both Denmark and Greece, and he had to give up his claim to the throne within his own family when he married Princess Elizabeth. For a long time, he had been demoted to Duke and Royal Consort to the Queen, and he was perfectly fine with this. He loved Elizabeth, and was willing to do what it took to make her happy and have their relationship work. He was eventually named "Prince Consort," but a few years later after WWII.

This has also played a part in what titles Catherine Middleton and Meghan Markle got when they married Prince William and Prince Harry. Marrying a Prince does not automatically make you a princess, not even if you have royal blood. Instead they were both made Duchesses, women with royal ties through marriage.