Really, the reason is quite simple.  Fad diets fail to grant their subscribers longtime success because of their very nature, namely because of the fact that they are diets, not lifestyles.  A healthy lifestyle is the only way to achieve long-term weight loss and fitness, and fad diets almost exclusively exist in the realm of unreality when it comes to people being able to follow the diet for the rest of their lives.

One such fad diet, which is terribly flawed, is examined in the article below:

http://www.printnpost.com/articles/4470/1/One-Fad-Diet-that-Wont-Get-You-Results/Page1.html