A backplate and wing needs to be custom fitted to the individual, and you actually need to know how the hoses, etc, are supposed to be routed. With a typical vest, you try on a small, medium or large, and if one of these does not fit too badly, that is the one you will buy--it is not typically a perfect fit, but for most divers in the mass market, it is all they know, and without experiencing a custom fit, it is good enough for them.
Also, the vest style will have all the routing and setup handled with very standardized straps and there is no real knowledge required to gear up a new diver in one-- in other words, a dive shop can hire the kid from Publix to sell gear in the shop, after a 10 minute tour of the shop's gear, the kid knows what he needs to, in order to sell one of the vest BC's to most new dive shop customers.
This would NOT be true with a BP/wing--and if it did occur, the liklihood is that either the consumer would need to discover proper configuration , custom fitting and set up on the Internet, or they would end up quite upset and poorly configured.
However... Once the Bp/wing is properly custom fitted and hoses routed correctly, and the diver is correctly configured with this, the actual "diving" part is incredibly easy and effortless. Again, if the instructor was to teach new non-diving students, it would be necessary for him to have used bp/wing himself, and be familiar with the proper configurations, and the concepts of air share via long hose and necklace reg for the donating diver--this being the intended scenario for the bp/wing---cobble a short hose primary and traditional octopus to the bp/wing, you don't have optimal routing and strapping for this, and the system does not work as well....
If a shop does not like Bp/wings, and there instructors have never used them, even if they bought a bunch of Halcyon gear to sell, they would have a self fulfilling prophecy going on which would sabotage any potential for the bp/wing concept in the shop...