The simple answer is because at the moment it's the one thing that will pretty much definitely help people do better at blogging and at making money from their blog.
Does the book guarantee untold riches?
Not quite. In fact although it has a pretty good set of solid steps you can easily take to get blogging and making money it misses out one thing.
That one thing is the massive emphasis you need to place on choosing the right niche.
You can get a set of keywords that have 5,000 searches per month and you can have keywords that have 5,000,000 per month.
The 5,000 search niche might be ok to choose if everyone buys compared to, say, 1 in 100,000 buying from the 5,000,000 search.
However, all things being equal...
The money you make depends on the traffic you're exposed to. Which in turn is linked to the size of the niche you have.
So it's essential you choose the right niche.