I think the internet has had a large impact, which might be why things began to change in the 1990s. It has gotten progressively harder to keep membership away from dissenting views and damaging information, as the internet went from a large and sloppy repository of information to a much more streamlined and accessible medium for learning anything you want. It is almost impossible to keep the sheep ignorant of your past when there are websites where you can read actual scans of original books and magazines. The dirty laundry is everywhere now.
And for prospective witnesses, it's a quick web search and suddenly you know a lot more about the JWs than they would like you to know. How many interested people became disinterested after a couple of hours of research?