This article was posted on CNN Health this past Friday. I meant to post it then, but forgot.
These are the top 5 reasons:
Confusion over which approach is best
Not losing weight fast enough
Bloating
Lack of support
Slipping up on difficult days
Each topic goes into a little detail. It's worth reading. The article's author has a Master's degree in nutrition and has a book out that I'd like to read.