Maybe because when Lyon and Healy began making harps, most harpists were men. Wurlitzer, I believe, used letters to designate models. Seems that tools and machines used primarily by men are identified by numbers, i.e., Boeing 747. But then again, Singer sewing machine models were given numbers, i.e., 201, 301, etc. Could have been a fashion thing too. Maybe the trend right now is to give products a name.