Sounds like peer review? It's difficult to find people who can provide an informed analysis of an activity unless you choose from amongst people who can do that activity themselves
If NYBR reviewers mostly review books by other NYBR reviewers, while ignoring the tremendous amount of other quality authors and literature, that is likewise problematic.
Unless one speciously defines the type of book that NYBR would review as its own genre (analogous to a researchers field), shrinking the candidate literature down what NYBR reviews.
The comparison to peer review is perfect, because exactly this happens across many areas of research.
Any area where the pool of potential reviewers is small enough that everyone (sort of) knows everyone else, the process tends to get corrupted by concerns over personal networks, careers, grant funding, etc. etc.
Much as prestigious book review cliques get corrupted by networks, careers, access to publishers, etc. etc.
The point being, becoming a peer of that kind is impossible to do through academia, so some people consider it "clubby". It is, perhaps, that these people have been both commercially successful in their niche and influential that confers that peerhood, not shared social ties (although I'm certain those form too).