Because a calorie is a calorie and you only need so much protein. Your body can use simple carbs and fat just fine for energy.
Mike Mentzer knew that and did pretty well as a bodybuilder.
Explain to me how a calorie is a calorie?
Protein is nitrogen based, fat is carbon and hydrogen based, as are carbohydrates. How do they not differ in the body?
Just because you can use something for energy doesn't make it the most effective for that role.
Obviously protein can be used for energy, and in reality, the human body doesn't even need carbohydrates to survive, but the conversion of protein to useable glucose, or use of BCAA's directly by the body is a much less effective method of energy production than using glucose.
simple sugars are a great source of energy, and in fact, all carbohydrates are broken down to monosaccharides when they are absorbed through the small intestine, but when ingesting simple sugars, you have to deal with varied levels of blood sugar.
Glucose is only an effective energy source when it is available, so shortly after ingesting simple sugars, you're back to lacking an efficient energy source.
It's fun to say that a calorie is a calorie, and it's much more fun to eat that way obviously, but as far as an applicable use for bodybuilding, I don't buy it.