Probably because crowns and closely related, tiaras, are seen as feminine when used in general imagery.
The image of the older Elizabeth on the £5 coin's reverse side shows her wearing the the Girls of Great Britain and Ireland Tiara (reputed to be her favorite, and bought from the Crown Jewellers Garrard in 1893 as a wedding present for Mary of Teck, later Queen Mary), while the younger is wearing the George IV State Diadem (as seen on postage stamps).
At some point he'll probably be wearing St Edward's Crown, used in coronations - but even that isn't a cast in stone certainty.
https://en.wikipedia.org/wiki/St_Edward%27s_Crown
Can you imaging balancing that... thing on your head?
Then there's the Imperial State Crown, used in the opening of Parliament (and at the end of the coronation).
https://en.wikipedia.org/wiki/Imperial_State_Crown
But Charles doesn't seem to me to be a "crown person".
Not that he'll take to wearing a cloth cap to open Parliament, but I suspect he'll just stick to a military uniform or a suit.
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Giraffe: a ruminant with a view.