To display the chain. Just search for used bicycles on some online marketplace and you'll see the majority of them are shown with the chain facing towards the camera.
It's actually much less complicated that even that. Consider that there are only two possible sides to show, and one of the sides has the drivetrain on it, whereas the other has basically nothing of importance. It's not much of a choice at all.
Can we have left-chained bicycles, please? (if nothing else, just for the novelty / fun aspect)