When I was younger and "in" I used to ask people at conventions that I got chatting to, why they joined. The variety of answers I got, looking back, shows me that the reasons people joined were as varied as the reasons people leave.
The funny thing is, not one of them ever said "Because I became convinced JW's had the Truth" which is what I wanted to hear, being full of doubts myself. Obviously indoctrination once in does its work, but getting sucked in is for some very strange reasons. I don't think anyone said to me either that the JW was so convincing in his/her "presentation" was a reason.
I now am surprised they make any converts, but the ones I have observed locally who converted all seem to be vulnerable people, and many are of low education and low self esteem. The vast majority, 98% or so, of growth locally is from born-ins and JW's moving in from elsewhere, from the "territory", only a handfull in the last decade across four Congregations !