One important reason adoption of agile project management approaches took so long is that, contrary to many people's understanding, they are relatively recent. The Agile Manifesto is over a decade old, as noted, but it offers little for agilely managing a project -- this generation of work was about agile processes for software development (a very different animal). It is perfectly reasonable to manage a project for agile software development in a traditional PMI waterfall approach.

That is not to say there hasn't been a need for cultural change.

Although the PMBOK has long included statements to the effect that there are several approaches to organizing a project plan, there has been a strong cultural preference for high-structure waterfall approaches within the project management profession. I suspect this is largely because of the early predominance of civil engineering projects, where these work well.