There are three possible reasons I can think of. 1) Probably the most likely, is related to the electronics. Stuff got shuffled around. This was probably more

There are three possible reasons I can think of. 1) Probably the most likely, is related to the electronics. Stuff got shuffled around. This was probably more true in the mini and nano designs, where space was even more precious. Bigger iPods might have simply been made to match. 2) Having the headphone jack in the middle might have made it difficult to determine orientation when the iPod was in a holster. 3) Cynical, but the location of the jack also made it easy to tell at a glance if the iPod was the 5G video version or not, as a point of visual differentiation.