Yeah, what Adami said, plus the fact that at the time that order was proposed by fans, we only had the PT and OT, and I think most would agree that VI is a better "ending" to the saga than III if you ended with that. VI feels climactic. Seeing that, and then going back and doing I-III makes the ending feel kinda..."meh, the story was good, then then took a serious decline in quality over the last half." And VI as an ending fits tonally with both the OT and PT, IMO, so it flows well from either V or III.