Why don't divers buy more BP/Ws? Because they are never introduced to them in class.
In my OW classes, I show all kinds of BCD choices and discuss the pros and cons. I show them my own BP/W, DIN valve regulators, etc., so there is no anti-BP/W bias in my presentation.
I show them all manner of alternate air setups as well, including long hose and bungeed alternate.
I am unaware of any case where one of my former OW students bought a BP/W. I am unaware of any of my former students buying a long hose configuration.
Although I do not see all my former students buy equipment, I am very much a part of the ongoing discussions in the shop about the sales of this gear and the fear that we are not selling enough to justify an inventory.