Interesting question; could you share more of the context, or a link/reference to the article itself? There is a tradition of considering the Carolingian ancestor Pepin of Landen as a saint, though the Roman Catholic Church does not attribute any official credence to that popular acclamation. Absent further context to guide the search, he'd be my guess: tonsuring (a ritual cutting of the hair in a specific pattern; in the West, often shaving the top of the head while leaving a ring of hair around the sides) was for those who took religious vows; that is, ordained clergy and professed monks. Pepin was neither priest nor monk, so it would make sense that he was never tonsured. Why the author of your article says the Carolingians prized this detail is, without more information, unknown to me.