Arbitrary, indeed. And so was Mr. Schrader's decision to exclude documentary and avant-garde films, and ultimately limit his selection to one film per director -- somewhat self-defeating, especially in the latter case since he'd so brilliantly set forth his criterions for the films in the accompanying piece (lamentably not available online in its entirety at this point). 

Your objection is understandable, but I think enough time had passed for him to reflect back without any major biases (something like this can't really be proven one way or the other). In any case, he hasn't officially worked with Scorsese since 1999's Bringing Out the Dead.