The over-processed, white bread diet that Americans eat is the primary cause of obesity. That coupled with a lack of exercise. High protien diets are not rocket surgery and can produce marked weight loss.
In all things moderation. You need not go from one extreme to the other, simply cut back some processed carbs in favor of higher protein will have dramatic results as well without the health risks of an extremely high protein diet.
A close second is the copious use of corn syrup and high fructose corn syrup in just about everything. That hidden sugar and calories could blow just about any good diet, whether it's Atkins, South Beach, etc.