One reason is that the USB port on my garmin is not waterproof. Furthermore I read several posts here on the forum that the USB is not good as a permanent solution.I am confused I have a Tom Tom GPS and just plug the GPS into the USB port. It is the same cable I would use to connect my GPS to my computer or a charger. I just bought a longer USB cable about 6 feet and was fine.
I think I will keep the connection directly to my battery, I've had this for almost 20 years on my Harleys without any problems.