I wouldn't think it was a conscious thought other than like you said, some don't fit horizontally, i.e. model 8 pancake style. There would only be a couple of sheaths for smaller models that would have fallen into the "don't fit horizontally" category. Me and Hunt had this same conversation years ago.

In any case, I doubt Bo would have had made a decision based solely on appearance, if in fact he was involved at all, but based on uniformity and efficiency.