1. Many people buy these camera's that never use them, or use them a few times but realize editing videos takes longer than they hoped it would. So after a while of non use, they sell the camera.
2. They built a good product, one that lasts and is therefore good for the used market. People are comfortable buying a used GoPro because they know it's rugged and well built.
So GoPro's have a good resale value and therefore many people that might normally buy a new one, instead turn to the used market which doesn't help GoPro at all.