A king's wife is a queen, so why is Elizabeth II's husband still only Prince Philip?
Diana would have become queen of England as a result of marrying Charles (assuming they had stayed married). So why, despite the fact that he is still married to Elizabeth, is Philip still a prince?
There are queens and there are queens, you thilly. The kind of queen Diana would have been is a "queen consort." This vaguely illicit-sounding title merely means she would have been queen by virtue of having married Charles, not queen in her own right. Were she actually the head of state, as Elizabeth II currently is, she would be the "queen regnant." In your dreams, ho, you can just imagine Liz thinking. (Well, I can.) But now Di won't be queen anything.
One might ask: why can't Philip be the king consort? One would not be the first person to raise the issue, though it wasn't Philip that it was first raised about. That'd be Prince Albert, and the one who did the inquiring was Queen (regnant) Victoria, to whom Albert was married.
For years Victoria endeavored to have the title king consort bestowed on Albert. But there was a lingering feeling that while it was all very well to have a queen subordinated to a king, it would not do to have a king subordinated to a queen. Eventually Albert was named prince consort. The title became so thoroughly identified with him that Prince Philip hasn't dared to assume it, and he remains an ordinary old prince. Though I suppose it's not like anybody's going to get him mixed up with the Prince Philip who delivers pizzas.