My father, who was never a JW, said it well: "People have got to believe in something." That's the bottom line of it.
Many people have been abused in their life, neglected, and having automatic friends just by joining this group that seems to be making a difference, with seemingly good, honest, decent people? (Granted, there ARE good, honest, decent people in the religion.) It's a chance to belong. To feel loved, accepted.
My mom grew up with an abusive, alcoholic father who literally tried to kill people when he was mad enough--thankfully he never succeeded. When she started her study with the JWs, said father was, I believe, dying of cancer. Simultaneously, she heard voices that told her Jehovah's Witnesses were the ones who knew the truth about them. So, she called the KH, started a study.
She knew little about the Bible, didn't even understand her own beliefs as a Catholic. So, she was easy prey--no need to question anything, and without any other standard to draw comparison with, the alternatives were non-existent.
In the absence of alternative points of view, an idea presented persuasively can influence even the intelligent. As long as you don't bother to check the facts, you're prey to any idea. With a more specific program of manipulation, it's just that much easier to accomplish.