The rough side is just ‘unfinished’ just like the inside of a leather belt. Also, a cowhide is ‘split’ in half these days and the inner layer is used to make ‘collagen’ ‘casings’ for sausages!
There is no reason for a manufacturer to spend time and resources on finishing something that really doesn’t need it!
I do have a Vintage Strop that is finished on both sides, but they are finished differently.