I think the vast majority of divers are 'occasional, recreational' divers - as in, those who perhaps dive 5 or 10 dives a year. As such, they're far more likely to simply take the word of the LDS and happily dive a poodle jacket or simple back inflate BC.
Scubaboard users, by very nature of the fact that they're taking the time to research the sport on the Internet et al, are somewhat more serious about their diving, and tend to do more research. Most LDS's won't push a backplate on recreational divers due to the lower profit margins and the mistaken belief that it's a technical rig.
Since most scubaboard users are somewhat more serious about their diving, they're more likely to wear a better system.
*** The above statement is a generalization. I'm sure there are going to be folks who say "I'm a serious diver and I don't dive a backplate!" Don't get indignant - I'm merely pointing out why there are more BP/W users. It's because we know better.