High protein diets of any kind are easy to lose weight on and people who do so often become evangelists.

Ketosis works for burning fat, but long term such diets are rarely followed. Humans are omnivores and are highly adapted and motivated to search out and eat a variety of foods.

Extreme diets like the fruitarian diets and the various all meat/milk/egg diets out there come and go as fads, but they don't ever find really large groups of people practicing them for any significant amount of time.