OP: I'm not sure, to be honest. It seems counterintuitive, and it may vary state to state. Speculatively, It could stem from most people's thinking that "you have to be 21 to buy a handgun," whether that is correct in their state or not, and writing laws based on that belief without checking the other law first. It could be because your state, at one time, did restrict handgun ownership to 21+, and changed that without changing CCW laws.
Or, it could be that it's a simple way to limit practical carry to those 21+ without dealing with the political problems of trying to raise the age of ownership for handguns. Your guess is as good as mine (I'm in the same boat you are, FWIW).