Most of Agile adoption surveys indicate Scrum as the most used Agile framework. Personally I believe that XP was more robust but perhaps it came before the time was right but I agree that Scrum, with his simplicity is a good framework and can be integrated easy and efficient with PMBoK combining agility with governance.
The guide has only 19 pages, including the cover, content and acknowledgements. It clearly defines 3 roles only: Developer, Product Owner and Scrum Master. It also defines the framework "founded on empirical process control theory, or empiricism. Empiricism asserts that knowledge comes from experience and making decisions based on what is known".
What is known evolves with technology, the product development life cycle is dependent on technology. What worked in client server doesn't work anymore for cloud and mobile applications.
I use Scrum for over 15 years, championed and implemented the framework in the "perfect storm": small and medium software development companies and I love it. I loved the fact that we learned doing it and we had the freedom to do whatever worked for us.
I see a lot of "Agile" certifications for roles that have nothing to do with Scrum. What is the value of a certification for an empirical framework? What does it certify? Is a 3 days course enough to learn Scrum? It took us, in a software development company, more than one year to understand it and become proficient. While I see value in training after few months of experimenting with Scrum, I believe that self study and knowledge sharing between the members of the Scrum Team is more benefic.