Why are they always so sure? They are just clutching at straws and constantly making a laughing stock of themselves. Why don't they concentrate on the here and now and just accept that they they don't know when Armageddon is coming? (just for the record I don't believe there will be an Armageddon)
The cynical answer (and I am a total cynic when it comes to the Watchtower Bible & Tract society) - is that they depend on constant anxiety over the coming end of the world to keep their people in line, making contributions, and selling literature. When one of these prophecies fails, they fall back to one of the time-honored strategies: Change what they said they meant (like they did on 1914 itself, and later the "generation of 1914"), quit publishing it so that it eventually dies out (like they did with 1925), or simply deny that they ever said it (like they did over 1975). Their people are members of a high-control cult, and will pretty much go along with just about any new change in teaching.