One prevailing theory on why a Carnivore diet seems to be healthy for so many people is that it is the original diet we evolved to eat, we probably evolved the way we did because we ate lots of fatty meat for hundreds of thousands of years.
Another theory is that just eating meat is a very good ‘Elimination Diet’ where irritating foods are eliminated from the diet, often times this mitigates or helps heal medical conditions such as Depression, Anxiety, Arthritis, Psoriasis, Type II Diabetes, and many other illnesses,
Still another theory is that people tend to reduce body fat and increase muscle mass on a carnivore diet, this alone can account for many of the improvements in health and fitness.