Sugar becomes toxic after too much time spent in the blood. There's a reason why insulin spikes in healthy people are around 3 hours. It starts becoming a toxin after that.
IMO, my common sense tells me that the body burns energy in order or toxicity.
Alcohol is the first macronutrient to be burnt. It's highly toxic and is quickly broken down into acetates for energy. As it is poison, all other metabolic processes are stopped until it's out of the system.
Carbohydrate is toxic when elevated in the blood as sugar for too long. Carbohydrate stores in the body are also the most easily depleted and refilled, plus they're rather small. They're like nitrous tanks on a race car - not necessary to go fast yet they're a big turbo boost, but they'll start breaking stuff if they're used all the time (hey, that's a pretty good analogy I just came up with!) They're not acute poison like alcohol is, but they can be problematic in large quantities.
Fats are next on the list. They are largely harmless (minus trans fats and too many polyunsaturates). Since they're the most stable, it is therefore in the body's best interest to burn these as fuel last. The body is okay with just letting this type of energy just sit around, so it's not going to use it unless the other more toxic fuels are removed first.
Protein is actually harmful to use as fuel. Your body deteriorates when protein is used, so logically this is the last choice.
At least this is how it goes in my head.