Most divers are recreational divers and have little to no need for a "tech" rig. Of those shops that have converted to tech, many have failed or have found themselves unable to meet the needs of the majority of the true customer base. Granted the tech wannabees around here will scream with anecdotal comments about how they have a great shop that caters to GUE people.
Think about it really. A typical dive shop certifies new divers year round, of those, a portion will purchase BC's and regs, the rest will be content with renting gear on their infrequent trips to warm places.
As convinced as some might be, a BP/W is not for everyone, not even most people. Any dive shop owner in their right mind is only going to sell the equipment that people ask for, and they don't ask for BP/W and a 12' octo hose. The recreational diver that has tech gear may be happy with it, but that is equipment that is certainly not designed with the typical diver in mind.
Why are BP/W more common on this board? Merely a difference in demographics and psychographics of the users of this board. Why do you not see that equipment more often at the beach? Because the equipment is not designed for the average diver, and furthermore, because the certification agencies associated with the equipment place themselves in that position. GUE and the DIR niche (key word niche) have characteristics* that make them less appealing to most certified divers
*on average in general