Sheesh. This question comes up about once a month, and this is the usual sequence:
>..."Why not?"...question.
>The beat-down begins:
>Never done it that way
>It's too difficult to close valve/isolator
>OMG what about on boats
>The hoses are longer
>More breathing resistance
>Non-standard
>You will damage the valve, you dimwit
>This is easier to tangle because the valve's on the bottom
>...and on and on...
>OP, suitably bludgoned, whimpers away.
As someone who puts on an inverted cylinder about 3 times a day at work, and uses it, I can state that there is absolutely no doubt that inverted cylinders are:
>Easier to open/close valves. I can even reach around with my left hand and close the (right-handed) valve, even with the anti-close ratchet our valves have.
>Faster to switch out cylinders. There is just no comparison. Spin, click, pull out the cylinder. Shove in a new one, click, spin on the valve. all done in about 45 seconds, no kidding. I feel handicapped changing out a regular cylinder, let alone doubles.
>More tangle-resistant. Really, you think that those 1st stage(s) and valves behind your head are cleaner than the rounded butt end of a cylinder?
>More durable. Yes, my work airpack gets dropped on it's valve once in a while. Nothing happens except a sheepish look.
>Easier to put on. Yes, I can hear the gasps from BP/W users now...this is a statement from someone with 3 (edit: now 6) plates in the house, and there is no doubt in my mind, when looked at from the totality of donning the system, BP/W comes in second best.
I'd suggest that for someone to discuss the pros and cons of inverted cylinders for diving, you should have equal experience in both configurations. All you have to do is try it in zero vis, and you'll be sold.
Back to the OP's question: I believe the biggest reason that you don't see inverted cylinders, despite their advantages, is pure inertia of "that's how we always done it".
All the best, James