The only reason I can think of is that I've never seen a machine binding that is as perfectly mitered and sewn as a hand binding. I personality like to hand sew a binding, and I usually think of "hand" as a four letter word! And I do machine bindings sometimes when perfection is not a requirement, because I can't get a machine binding to look as nice as a hand one. But if someone can get a machine binding to be perfect, I see no reason why it shouldn't be judged accordingly.