I believe rats have a higher muscle to fat ratio than mice. Also, it's easier to feed one appropriately sized rodent than multiple rodents or more frequent feedings. My BP should be on rats by now, but absolutely refuses f/t to the point of losing weight, and the only available options within any sort of reasonable distance from where I live are adult mice and large adult rats (which she is not big enough for), so I feed more frequently than most people. Some BPs are just dedicated mousers who won't take rats for whatever reason. They won't suffer from deficiencies or anything on a mouse diet as long as they get fed enough, but it's easier and a little healthier to feed rats.