Why does the 14th mention ‘naturalized citizen’ and draws a distinction between them and ‘citizens’?
Because they aren't the same thing. Former slaves were not "naturalized" in the conventional sense. They were "naturalized" en masse by passage of the 14th amendment.
Now a question for you. Why aren't the words "natural born" right next to the word "citizens" in the 14th amendment? Was ink too expensive?