According to the article, Zusak's publisher, Random House, credits " book clubs, community read programs, and passionate word-of-mouth recommendations."
In other words, the publisher and author targeted the niche audience for this book. Notice they don't credit Walmart, prime placement on Amazon's front page or a huge advertising campaign for the book. Too often, authors are looking for the whiz-bang flash of a splashy ad in a newspaper, or pin all their hopes of a bookstore chain carrying their book in every outlet in the country. There is much to be said for word-of-mouth advertising.
The author himself is surprised, saying he didn't think anyone would be interested in reading the book. He simply writes for the sake of writing. He has some pretty interesting comments beginning at 3:26 in the interview below.