I always wondered about that, even back in the 80's when JWdom seemed bold, confident, and strong. I remember a Watchtower article (late 80's) that instructed JWs to immediately throw in the trash any information they might receive in the mail that was against JWdom.
I always wondered what it was that we were supposed to be so scared of and why we should be so scared if we had the truth and had the most powerful being in the universe backing us up.
Now, I know. I now know that back then their doctrines were being soundly questioned/challenged by such individuals as R Franz and Carl Olof Jonsson. JWdom began to realize more and more that its doctrines were not solid and couldn't be defended. It became more and more scared of apostates and material outside its own publications.
And, now, today, it has far more reason to be scared. As Hanged Man said, the gig is up. As steve2 said, JWs are ill-equipped to defend their faith. In fact, it can't be defended.
Other than their basic doctrines (Trinity, hellfire, condition of dead, future of earth, etc.), I don't even know what their beliefs are anymore. They seem to have abandoned all the deeper stuff - the Franz "deeper" publications, the Revelation Climax book, etc.
The org knows it can't defend its beliefs and that there is ample evidence of its crazy history, deception, faulty doctrine, etc., so it resorts to instilling in its members the belief that outside material and opponents are from the devil and will corrupt their minds. It's a cult, and it uses cult tactics.
JWdom can never again be bold and challenging. It is running with its tail between its legs. It uses argumentum ad hominem (denigrating any who challenge it without even addressing their arguments). It shoots the messenger so it won't have to address his message, of which it is terrified.