Very few of the witnesses I knew, or members of any organised religion really, could fairly be decribed as stupid or mindless.
In some ways it seems easier to understand born-ins as they are shaped from birth to fit in with the this religion. They usually know no different and perhaps many won't consider alternatives unless some life-event makes them consider their position. This seems to me to be human nature - as mentioned above.
Those who are recruited usually join for a reason - either already religious (as was I) and therefore already accepting the Bible (say) as an authority- from there it can be an easy task to demolish one set of beliefs and replace them WTS doctrine. In my own case, I was young, vulnerable and had no easy way to challenge what the witnesses were showing me. Bingo! Or they have some spiritual need and unfortunately JWs get to them with their slick patter, fear tactics and love bombing.
I think it is a lot easier now to research the background to the JWs than it was in the 80s and 90s. Also, my own thinking processes and critical thinking skills have definitely developed from what they were when I was a green 18 year old Catholic.