The world was bigger and more complex than the schemers imagined, and those who survived prevented or stifled subsequent attempts. No one has yet come up with a perfect weapon that would kill everyone, everywhere, on the first attempt. Nukes? One wouldn't do it. Several dozen wouldn't do it. I think even nukes would leave people alive somewhere. Disease? Same thing. There's always people who are immune.

In the future we'll simply build things bigger than can be completely destroyed. Small parts will be sacrificed to the inevitable act of carnage, but the whole will be bigger than the terrorist can plan for.