Its because Rutherford did not like beards ... bearded brothers reminded him of Russel's presidency and Jews (which were at the top of his long hate list) ... and that's the simple truth.
Its not in the Bible because it was actually seen as a disgrace for a man shave his beard. Never mind the God saw fit to have hair grow on men's faces....that's all superficial, right?
Its like asking where in the Bible it says collard shirts with ties are necessary to show respect for God. Jesus and the Apostles would be disfellowshipped if they were here on earth today.
Its specifically a man made up rule dependant on the whims of whomever deems themselves to be in charge and enslave others to those whims.