Just a guess, but charging by the old school barrel jacks requires next to no protocol support. You just need to provide the correct voltage, and supply enough

Just a guess, but charging by the old school barrel jacks requires next to no protocol support. You just need to provide the correct voltage, and supply enough current. Whereas with USB-C, you need to speak USB-C before it'll charge. EDIT: oh, and it gives you the ability to avoid potential security attack vectors.