One of the big reasons is that many of the current JW's who have been around for several years understand that they are much like Catholics in that they follow what someone else tells them is the "truth." The current "overlapping" generation doctrine is so convoluted and confusing that the rank and file JW cannot explain it in a clear and concise manner.
I have 3 JW's who work at my place of employment and not one of them can explain why Jesus chose the Society as the faithful and discreet slave in 1919. Most now just go through the motions to get by.