I suspect it's purely to make it less like;y to be confused, not everyone has 20-20 vision.
//With the exception of the knight, each piece is abbreviated to the first letter of its name: K for king, Q for queen, R for rook, B for bishop, P for pawn. Knight begins with the same letter as king, so it is abbreviated to either Kt (used in older chess literature) or N. "N" is used in this article. In 1944 Chess Review received many letters debating the change from Kt to N.//