Good question Blake, I had never thought about it.
Perhaps due to the solemn nature of the award, the reverse may be intended to be viewed by the owner and not by a curious bystander.
When he lifts the medal when being worn, the orientation will be good for him and the ribbon provides slack to do so. He turns it inwards instead of outwards.
I also think that using a pivoting mount would result in an occasionally reversed medal on the parade ground.
It would be interesting to know the generally accepted reason.