If I recall correctly, the Majesty uses the Game Changer switching system, where as the JP13/15 don't so the guitar have different power consumption. 3x AA batteries with the voltage stepped up to 9V, last longer than a single 9V battery. AA are also cheaper, are more commonly sold and less likely to be out of stock, than 9V batteries.
Hi there,
The preamps in the JP13/15 are vastly different than the Majesty preamp. The Majesty preamp is a fairly complex set of circuitry with electronic switching. The battery difference has to do with the needs of those particular preamps.