I'd bet on poor soil, cold climate, and isolation. If the earth is not high quality and if the summer is not long enough for high yield per square meter, then y

I'd bet on poor soil, cold climate, and isolation. If the earth is not high quality and if the summer is not long enough for high yield per square meter, then you need animals to turn grass in to food. You can't eat too many of your animals for the same reason you can't buy grains from outside - it is expensive and you have too little to offer as export goods.