Double As and triple As are cheap and plentiful. 9 volt batteries cost a lot, are sometimes hard to find. There must be a good reason you don't see AAs in guitar pedals. Something to do with 1.5 volt vs 9 volt transistor circuitry? I'd love to use a fuzz that took one or two AAs, for when I don't want to plug in the pedalboard. And don't get me started about 9 volt batteries in piezo-acoustic guitars.