It seems like the main reasons for the 46 BC dating are:

1- the founding of Arles

and of lesser importance:
2- pre-death realism (I really want to see a sideview...has anyone seen one?)
2b- immature features of Caesar's face that are more pronounced in (supposedly) later depictions
3- late Republican realism (though this by no means puts it within more than a broad range of years)