Simply because they probably liked it and they were likely at a point in their life that any basic healthy diet, better than they were previously consuming, would have had the same outcome. In other words, the trigger event in their life that led them to pursue weight loss and/or health, motivated them enough to finally stick to something. Be very clear, at the end of the day, "if you take in fewer calories than you burn you will lose weight”. Although the scientific assumptions by the Zone diets author are not generally accepted (see below), if the Zone formula accidentally hits your “sweet spot”, allowing you to maintain the lower calories, then it's a great way to go. In fact, we have a similar diet (40% carbohydrate, 30% protein, 30% fat) as one of our 8 options in the dotFIT program.
More info: Mastering The Zone claims most people suffer from insulin imbalances that cause them to gain weight. According Barry Sears, the consumption of protein, carbohydrate and fat in exact proportions corrects the imbalance and excess weight drops off. Living in the Zone also can prevent heart disease, diabetes, PMS, depression and cancer, according to Sears, who also claims that this plan can slow the aging process. Average caloric intake ranges from 1,000 to 1,700 calories per day, so certainly the weight loss is 100% contributed to the caloric restriction. In fact the recent study at Harvard confirmed this fact.
The plan recommends lean protein sources, minimizing consumption of saturated fat. The typical American diet is not far from the 40/30/30 ratio of carbohydrate/protein/and fat.
Carbohydrates such as grains, fruit juices, pasta, starchy vegetables and bread are not prohibited but serving sizes are minute (1/4 c. cooked pasta, 1/8 c. baked beans, one-square-inch of corn bread). Choices, combinations and portions can be odd and unappetizing. For example, a snack choice includes two hard-boiled egg whites, half an apple and three almonds. Most endurance athletes would find this diet inadequate for their energy needs.
Because the plan is based on low calories and strict combinations of protein, carbohydrate and fat at every meal, long-term success might be challenging unless slightly modified.