The main reason is most people know nothing about them. I was on a plane the other day, talking to a 45 year old woman sitting next to me. In Australia about 1:330 people are JW's, yet she had never spoken to one before.
If a religious person arrives at a door wanting to give away a magazine most people are not going to be interested, let alone contemplate changing religions. There are 35,000 Chrisitan sects, and JWs are just one small drop in that bucket.