Why aren't there more terrorist attacks?

Bruce Schneier asks, Why Aren't There More Terrorist Attacks?: If terrorism is so easy,

why aren't there more terrorist attacks like the failed car bomb in New York's Times Square? ... After the enormous horror and tragedy of 9/11, why have the past eight years been so safe in the U.S.?

There are actually several answers to this question. One, terrorist attacks are harder to pull off than popular imagination -- and the movies -- lead everyone to believe. Two, there are far fewer terrorists than the political rhetoric of the past eight years leads everyone to believe. And three, random minor terrorist attacks don't serve Islamic terrorists' interests right now.