Even when i was "in" i used to say "this religion attracts nutters".
If someone you love dies, be it parent, child, sibling, friend... and someone sells you a fantasy of seeing that person again on earth and you living with them again people sometimes buy into it. Because they want it to be true.
Also, a lot of people have respect for the bible, or think that it's holy even though they dont follow it. So when a cult comes along and shows them that the bible really says x, y and z and not many people know this you feel special and enlightened.
Im a 2nd generation born in, although my grandad did convert after my dad did. My dad came into it because he saw positive traits such as honesty, unity and all those smiling faces. My mum came into it as a 15yo girl after being abused at home, growing up in foster homes from age 5 and no one ever caring about her. Along comes the JWs and instant friendships and suddenly a purpose and goal in life. This is why mum will never leave. Even if she see's it for what it is. The org is her parent now, and it has been since she was 15.