High Carb diets work in countries where poor people are physically active and are fighting not to be in caloric deficit. Whatever carbs they take in are burned or stored as glycogen.
In a rich country, like the U.S, where poor people eat excessive calories, and are lazy, high carb is a disaster. Glycogen stores are always filled to capacity with blood sugar still high. So, the body is forced to use carbs as its primary fuel source. Whatever fat is eaten gets stored as fat. The end result is obesity, diabetes, and heart disease.