When I had a google it turned out he was really peeved that for 50 years he was Prince Consort rather than King (although, apparently, didn’t blame his wife for this). It does seem a bit daft in this day and age.
This is something of a hobby horse of mine.
What it comes down to is that the legalities of something, might not reflect the way something is perceived according to the language we use. In this case, in the context of gender.
So, a woman married to a male sovereign (king) can be queen because our communal understanding of language is that queen is always inferior to king.
However, a man married to a female sovereign (queen) can't be king because even if officially, legally etc the queen is superior, our language means that king will always be perceived as coming first/being more senior. We don't refer to the "Queen and King" for example. So, theoretically, Philip of course could have been King Consort, but I think everyone would have seen that as confusing because the default assumption would always be that King is the more important one and therefore it would have been considered something to avoid.
Now, the big question is whether the language can and will change. At a simpler level we are slowly seeing shifts in perceived genders in language. For example, if I tell DC they are seeing a doctor, they don't assume that doctor is male (in fact, they generally assume it's a woman as their GP is a woman, the only doctor friends we have are female and we have cheated somewhat by referring to all medical personnel as "doctors" including those who are more likely to be women such as nurses, health visitors, occupational therapists etc...) [Much to my frustration, I, on the other hand, DO still assume it's a man and have to consciously think about the pronoun I use.]
If for some reason Charlotte was to ascend the throne in say 40 years, it would be interesting to see if language and the implications/connotations that go with it will have moved on by then so that her husband (assuming she has one) can be referred to as King without there ALSO being an automatic assumption therefore that he is the more "important/senior" sovereign.