It's just like any form of marketing. It's can be difficult to prove that somebody bought a book because of the book's website. From the tone of the article, it

It's just like any form of marketing. It's can be difficult to prove that somebody bought a book because of the book's website. From the tone of the article, it doesn't seem like many publishers have really tried that hard to do a really good web analytics implementation to find more information. Also, the website could be really ugly or offensive, which could scare off a potential impulse buy.