We have friends like that.
My theory is "because it works". It *sounds* so good, and when you have healthy, compliant children it produces almost instataneous results. And it looks good to other people, and it keeps you that much more able to "do the Lord's work" because your children are happy and compliant most of the time.
Because honestly, the people I know who use it have compliant children that don't require that much effort in the first place. Not that they're easy all the time, of course - but nothing in comparision.
I don't know anyone who has stopped using Pearl who has compliant children. In fact, most of them deal with children suffering from some degree of mental illness. :/ Whether that's impacted by the Pearls' teachings or just why they stopped, I'm not sure.