Follow the money......
Most dive shops conducting entry level training tend to be philosophically at odds with those advocating BP/W's, because the majority of BP/W users and the agencies with which they affiliate also advocate a paradigm of dive training and entry requirements that are incompatible with the dominant dive shop business model. Without getting into who is right, BP/W enthusiasts tend to support a philosophy called DIR which emphasizes rigorous training that requires a large commitment of time, effort, and personal fitness. It's a philosophy that would probably exclude 60% or more of currently certified divers from the sport. The dive shops, of economic necessity, tend to lean the other way, because otherwise, they'd lose 60% of their business.
In addition to that, jacket BCD's cost more, don't last as long, and with their greater complexity, are more likely to require service. They are also more likely to be replaced before they've expended their useful life in pursuit of the latest "bold new graphics" or new gimmicks. There's not much a manufacturer can do to make a BP/W "new and improved." Plus, most BP/W manufacturers sell direct on the internet, effectively reducing the markup a dive shop can get on them.