It's success is simple: it caters to the egoist in everyone.
Anyone who thinks that you can wish your problems away through "positive thinking" is engaging in self-delusion and laziness. It helps you delude yourself with the idea that you don't actually have to do anything, just think it. If it didn't work, well, you didn't think hard enough obviously, right?
The corollary not explored is that apparently all bad things happen to you because of weak/bad thoughts. If your mother died of cancer, is it because you or someone else wanted her dead of cancer? Who's ego wished that up?
Positive attitude, from a psychological not spellcasting POV, is definitely an important component of success, but it is not the only one. The Secret takes that and magnifies into a solve-all mysticism that is absurd.
It's a snake-oil.
Of course you need to set goals and think hard about how to achieve them. But seriously, that's not news to anyone, is it?
Read this more well put criticism about the book.
If you need a good self help book, I recommend How to win friends and influence people by Dale Carnegie, Winning by Jack Welch or anything by Peter Drucker, especially his writing on "Managing oneself ". These are people with actual working scientific theory behind them.