this is the correct answer.
But not the fun one, and in a way, it is incorrect too. The question was what prevented them
in the timeline of the story? Within the story, if your going to play the game and not be a literal moron, the attacks should have happened and didn't, thus something changed. Within the context of the topic, the fact that it was published before the attacks occurred is completely irrelevant.
I'd like to think that maybe the yeerks had a small base wherever Ossama bin laden was and he was made a controller. He was still a terrorist before that, but after he was freed, maybe he decided that enough was enough? Maybe he was touched in a personal way and called off the whole operation, choosing to focus his hate on aliens?