Something troubling happens in the organization. Brother Sheepy wonders about troubling information, but the idea that this means that this is not "the truth" is scary. Brother Sheepy would have to leave and loose all friends and family, possibly his employment. He would have to admit to non JW acquaintances that he made a huge mistake and has been wrong about everything. The cognitive dissonance is strong, the idea is unacceptable to him, so he reasons that it is just a trick of Satan, it must be a trick of Satan, the brothers said that would happen. It's far easier to grasp at that thin explanation than question everything you know, it's human nature.
Even in cults where the leader made a specific date prediction that turned out not to be true, most people stayed. One group spent the night on a roof, convinced the end was going to happen. In the morning everyone got down off the roof and pretended nothing happened, most did not leave the group. That's why 1975 did not result in a mass exedus, but a trickling away over the next twenty years.
You have to already be questioning for these things to have any effect. If you think back to your own questioning, did the first bit of suspect information cause you to go, or did it take some time? For me it was a number of things over a number of years, until one day I had an epiphany, a brilliant glimpse of the obvious, the Governing Body was just making stuff up, just like every other religion.