Sugar substitutes like Monk Fruit Extract aren't as widely produced. (There are other sugar substitutes, but among all of them, Monk Fruit is king: it tastes most like sugar and it is the least likely to cause digestive issues). If they were more popular, perhaps efficiency at scale would drive down prices. Also, carbs tend to be more shelf-stable than proteins and fats (though, beef sticks and nuts are solid exceptions to this generalization). But a long shelf life can help decrease the cost-- which is the case with staple grains like wheat and rice.