Humans are lazy by nature. The book 'The Secret' touched this part perfectly by promising money, wealth and fame without having to move a finger. If you want something just close your eyes and visualize it and it will come to you without having to do much effort.
People like to fool themselves in order to escape guilt. By telling themselves that they did their part, using the law of attraction, they can escape important duties without feeling guilty or admitting that they are escaping.
People can't deceive their minds with incorrect theories because that won't make any sense. However, when a theory is partially true or based on some correct facts, it appeals a lot to people and is easily believed.
The Secret Movie got popular, then the book followed later. People already got familiar with the concept and felt that they wanted to know more. The marketing strategy was brilliant.
The music, the intro, the people involved and so many other things were done perfectly in the movie. And because the movie emotionally connected to people, it made it much easier for the book to sell.
Products that result in controversy usually become more popular than products that don't, simply because the more people debate about something, the more likely more people will hear about it and the greater the chances it will become popular.
If a theory is controversial and at the same time there is no solid scientific data to prove it wrong, the debate might last for a long period of time. And because these debates fuel sales, the book will keep making more sales as long as people talk about it.
Not everyday do people find someone making such bold claims. While most self-help books repeat the same things, The Secret book came with a totally unconventional bold claim.
If everything happens as a result of our thoughts, then we must be in control of our lives. At least that's how so many people think about it. Most people like to believe they are in control and The Secret Book gives them something to hold onto.