I think people who become witnesses are looking for something outside mainstream religions that provides them with answers. JWs seem to do that superficially. There is also the lure of being part of a group and there are alot of nice JWs out there. At first glance, it seems like a nice bunch - it's not until you've been in a while that you start seeing all the major flaws.
New converts are usually vulnerable also in some way- in alot of cases I can think of- people have been ignored or feel isolated and here comes a group of JWs paying alot of attention to them and being super nice. Of course that's going to make an impression. And people might start feeling this false sense of "I know something that others do not" like you've discovered something that everyone else couldn't decipher, but here you are diligently studying the Bible and you're surrounded by a group of people who feel the exact same way. That's powerful stuff. You're all in this together- you suddenly have a purpose (preach!) and you know exactly what's going to happen in the future. Sadly, the euphoria doesn't last long....