Why do they buy in? Probably for the same reason that anyone becomes involved with a cult. And that is, initially, they come across as warm, caring, accepting, and this works for people who have problems in their lives, who feel that are alone, shunned, victims of bigotry, prejudice, or a myriad of other social and economic ills. And, people who doubt some facet of their existence. The JW’s are accepting. And it works.
Pentecostal and fundamentalist protestant churches understand the same thing. So, is it any wonder that only 22% of Catholics practice their faith today. Think of it, of the next 1000 fourteen year olds that your bishop (or any bishop for that matter) will confirm this year, 780 of them will have turned their back on the RCC. Why, well, if I had the answer, I might be pope. But the poor teaching and the reliance on methods of worship and teaching that have been used over and over don’t seem to work. And they don’t see that problem. I could quote the commonly heard definition of insanity, but why waste anyone’s time.
The JW’s succeed because they connect on a purely human, everyday wants and needs of people.