As much as it looks like you're just cooking the pork belly to the point of dryness, you're not. The cooking process is long for pork belly because of the amount of fat in the layers. This fat needs to break down and render so it's not thick and rubbery when you eat it. However, there is a point at which you can overcook pork belly and it becomes dry and tough. That point is when the fat layer has completely rendered down into nothing. Cooking a 2 pound pork belly for 6 hours on low in the slow cooker is not enough time to overcook it.