Originally, SCUBA gear was radically different and pretty much required tanks to be valve up when diving. the original recreational setup, the "Aqua-Lung" was a very primitive first stage that had a single corrugated hose attached at two ends of the stage with a mouthpiece in the middle to breathe from. the corrugated hose circled the divers head and the mouthpiece was an integrated one-way valve. The air came down the hose on your right, then continued up the left hose until your bubbles came out behind your head. This setup made it pretty difficult to orient the tank downward, unless you customized some reeeeeally long corrugated hoses to go up to your head, and that would be very impractical. When the "modern" first and second stages were developed, nobody thought to turn the tank upside down because "that's the way it as always been" syndrome caught on. That's actually why second stage hoses come around your right shoulder, because that's the way the Aqualungs were set up.
When I get back into tech, I'm totally inverting my doubles. All you need is a 9 foot hose instead of a 7 foot, and it's way easier for me to scratch my butt than grab my shoulder blades!