I think because anorexia is generally considered a mental illness, while being obese is more like a product of a modern society that loves large amounts of food with lots of fat, butter, cheese, sugar, etc. while at the same time living a very sedentary lifestyle. At least in America, being fat is almost the norm. Maybe not quite BBW levels of fat, but still. I think also anorexia is much more dangerous as starving yourself too much will lead to a pretty quick death, whereas medical issues associated with being fat is a much slower death.