Because BRITONS generally interact with their bicycles from the left, and will be in the habit of wheeling it, locking it, leaning it against walls, etc from that side[1]. Only the sort of weirdos you get on cycle forums think about the sprockety stuff.
Obviously if you're selling something, you should photograph it from all important angles. But these are lazy people, as evidenced by using Facebook for an eBay job.
[1] I've grinked the council on a couple of occasions to point this out, wrt the half-arsed installation of wheeling ramps.