I suspect Muddiver is correct.
Several folks I train with have purchased BB's in the past several months, and others have converted Derby Doubles to 4-shooters.
With time being a high priority on everyone training schedule, I think that trainers are doing more training alone. Many times, especially during the week, folks are unable to get together as a group to train, so a quickie training session after work for 1 or 2 dogs is a workable deal. That's where the BB's earn their keep for the individual trainer.
Incidently, I trained last Saturday with a guy the uses the Max 5000. We had a mark about 175 yards, that was extreamly difficult to see, even though there was a short white streamer on the Max bumper. This is not the first time I have experienced difficulty in seeing the Max 5000 bumpers. The owner stated that in his opinion, the Bumper Boy bumper is much easier to see. I agreed.