I don't know if there is one main reason but a major contributing factor I would say is a lack of clarity around what is "Agile" as originally intended by the folks who came up with the Manifesto back in 2001. Even among the originators, I think there were still differences of opinions on details and different aspects. What they did amazingly come to agree on was the set of values and principles that they held in common. These should be at the foundation of any Agile adoption effort and when they're not, things tend to go the wrong way.
You should note that this book is Bob's take on what Agile is. While his opinion certainly has weight, it's just one of seventeen. If you really want to understand the breadth and depth of Agile, you need to study what folks like Kent Beck, Ward Cunningham, Ron Jeffries, Andy Hunt, Dave Thomas, and the rest of that "Gang of 17" and their contemporaries think and how their thinking has evolved over the years. See where they agree and maybe more importantly, where they differ.
The best ideas are the crazy ones. If you have a crazy idea and it works, it's really valuable.—Kent Beck