Hormones? Or some metabolic process? Or is there no way of knowing?
Some initial research is suggesting that genetics may play a role. In his very interesting A to Z diet study, Christopher Gardner of Stanford found that within each diet group, some women did well and others not so well.
Researchers at a genetics testing lab asked for and got access to a little fewer than half of the original participants and tested their DNA for specific genes the company had linked to weight loss. They found:
Women assigned to a genotype-appropriate diet lost 5.3% of their body weight compared with just 2.3% among those not matched to genotype (p=0.005). Within the Atkins group, for example, those appropriately assigned by genotype lost approximately 12 pounds compared with 2 pounds for those who lacked the low-carbohydrate genotype. In the Ornish group, similar reductions in weight were observed among those appropriately assigned by genotype.
Gardner said the proportion of individuals who were genetically predisposed to the low-fat or low-carbohydrate diets is roughly 50-50, so a significant number of people will fall into each category. He stressed, however, that all individuals assigned to the diet groups were instructed to make healthy, wholesome food choices.
Probably because humans are not carnivores. Plain and simple. There is NO evidence for this to be the case.
Consider this from the paleo/evolutionary perspective. IF ketones and fats were the "preferred fuel" for the human body, and we evolved in an environment of scant carbohydrate availability, why:
Do we still burn glucose preferentially and find it so difficult to get into let alone maintain any significant level of ketosis?
Don't we immediately convert fatty acids to ketones (instead of only when we're low on glucose) for our brains to consume from the get-go?
Is skeletal muscle the biggest "sink" to dispose of dietary glucose, and most dietary fats go to adipose tissue first?
VLC is the adapted, nutritional state of necessity for humans in certain environment. There's little evidence it's optimal, and a ton of evidence for more optimal nutrition.
EDIT TO ADD:
It seems to me that the term "preferred" or "preferential" is emotionally charged for some. But the heirarchy of substrate metabolism for energy is well established and irrefutable. There are also metabolic disorders that exist in enough of the human population to where if they were advantageous we would imagine they would have been selected for by evolution. If humans didn't consume starch, why do we still have amylase? If dietary glucose was harmful, and ketones actually preferred by certain tissues as an energy source, wouldn't humans deficient in amylase have had a survival advantage and won out in the evolutionary process? This is what I was getting at. One would think that over thousands of millenia the enzymes for ketone generation would be so upregulated, and those for glycolysis so downregulated we'd just convert dietary fat to ketones and be done with it, even if a little glucose came our way, because our livers would be more than capable of handling that with its glycogen storage capacity.
Another thing to consider is how substrate levels in circulation are regulated. In large part, dietary fats do not contribute to fatty acid supply, that is regulated at the point of release from fat tissue. We certainly could have a similar process for glucose, where it went straight to glycogen and was released when needed. Glycogen is inefficient compared to fat for energy storage, but surely we could have double or triple the capacity without becoming prohibitively heavy. Glucose stimulates its own oxidation. If this were because it's toxic why absorb it at all?? We don't have enzymes to digest oligosaccharides, why do we have them for di and poly? Why have the metabolic path to convert carbs to fat but not vice versa? If we never ate much to begin with, let alone excessive amounts, what would be the point? Untreated diabetics survive short term hyperglycemia and what's wrong with a little glucosuria amongst friends anyway?
Hopefully this made my point more and not less clear :)
To expand on my original position, after you hit sufficient protein and EFA, I think the ratio of carbs to protein depends.
Ketosis can have a strong appetite bluting effect, and lowered carbs certainly seem to lead to many people feeling wired (at least in the short term).
As you get leaner, insulin sensitivity will increase and you will be able to handle more carbs and probably do better with more carbs.
Similarly, with increasing activity (especially high intensity), carbs will likely make you feel and perform better.
There is a huge genetic component to this as well.
So, like most things, the ratio of carbs:fat one feels best on is a bell curve influenced by many factors. People who give narrow responses don't understand the breadth of factors at play here. So experiment, find out what works best for you.
I think most people will be surprised that moderate amounts of each nutrient are close to ideal (20-50% carbs, 20-50% fat, 20-40% protein). As much as we like to think otherwise, we are not snow flakes, we are in the majority the majority of the time.
List of factors that should determine how you feel for a carb intake (and therefore, your carb:fat ratio)
Another fun fact: low liver glycogen levels can send a signal to the brain inducing hunger. So although ketosis does blunt hunger for some, having enough carbs to keep liver glycogen not empty seems to help some people.
Fundamentally, I don't think anything can be guaranteed to work if you've spent a substantial period of your life - particular growing up - doing something else. I think a lot of people would consider they did much better on low carb if they'd never had access to higher carb. But whatever you're doing, there are a whole range of factors that need to be handled differently. You can't just simply change the foods and not adopt different eating and exercise habits and expect it to work the same.