I think there are two components to look at here. First is that the Chair plays an outsized role in the student's development as a scholar and should know how to be a scholar, while committee members may be on the committee because of other relevant experience (e.g. a student writing a thesis on the medicalization of grief may wish to have an MD psychiatrist with DSM-V experience on their committee.) The other piece is that it sounds like Dr. Lexchin actually had published in the literature and was familiar with that process, unlike many other physicians. The same could be said about your average attorney, and possibly the JD that @chrisjm18 was referring to.