I feel it's a matter of design.
It's hard to explain but I suppose it's more natural that way. The metal part will close in on your wrist before you click the button folding the strap inward towards u and tucking the rest do the strap in at the 6 o'clock buckle.
And when u take the watch off, u can pry it open with your thumb instead of your fingers.
My humble opinion. I did thought about it and derived my own explanation. It feels more natural now, so I don't really bother why it ain't the other way.