Second, it literally has a chapter on Popular Music and another on Jazz Theory, and "Classical" is not found in its chapter titles. So I don't think your critique is accurate.
Third, history and time matter. "Theory" came first in "classical" contexts, with codified rules pre-dating even Bach. Jazz was a much later invention that built upon these and other traditions. Both of the Jazz theory books I used in my BA in Music Theoryhad many elementary topics already covered in Classical contexts.
If programming is an analogy, it's said that once you've learned your first language, you can learn any other language in a jiffy. Also, a lot of higher level learning is "learning how to learn." Maybe it's better to have a standardized "first language" that students can learn in depth, and then let them move into other areas at their own discretion.
I don't have a college degree, but for me, classical music lessons were a great preparation for learning jazz, and I've picked up pop, rock, and folk music on my own. In fact, on the instrument that I play, the double bass, gaining proficiency and experience in jazz without that background is a brutal learning curve.
Where this ties in to your point is that part of the reason they really don't want experts is that they rarely have someone whose job it is to teach middle eastern music. They might have a guitar teacher with some experience with oud, a flute teacher who can passably play ney (usually with awful intonation), etc. For most schools, these other types of music are "add-ons" that they have their existing faculty teach.
To me, that above ^ looks like the biggest gap.
To be fair, the book doesn't claim to demonstrate theory of the "world music" (sic, a phrase as bad as it sounds).
My (brief and shallow) understanding is that most scholars of music in the western world just do not have enough knowledge to talk intelligently about non-western genres. Exceptions exist, of course, but they are quite rare.
[Other reason is that there is a lot of gate-keeping. I've seen that in Hindustani/Carnatic classical studies, FWIW, which is one my areas of interest]