Obviously a big one is hunger. It's very hard not to eat when you're hungry. Some people can do it for a little while, very few can do it all their lives. And many of those with a serious weight problem have a genetic predisposition for an overactive appetite. Also there's boredom, just eating for something to do.
Many people eat too much simply because it's pleasurable.
For some the pleasure of eating provides an invaluable temporary escape from depression, anxiety, or stress. These reasons have strong psychological or genetic roots, and the Zone is not a cure for them. Indeed for some people they will be reasons why the Zone fails too. The Zone can help with them though, in a number of ways. Firstly, the Zone drastically reduces hunger.