Bruce Schneier pointed out a great essay, “Why No More 9/11s?” by Timothy Noah on Slate. It’s long, but worth the read.

The essay is divided into eight parts that discuss the various theories as to why there has not been a major terrorist attack on American soil since 9/11:

I. The Terrorists-Are-Dumb Theory: 9/11 was a lucky shot, being a big-time terrorist is hard.
II. The Near-Enemy Theory: It’s easier to attack the closer enemy (Iraq, Afghanistan) than far (US soil).
III. The Melting-Pot Theory: The US has been more successful in assimilating Muslims into society.
IV. The Burden-Of-Success Theory: It’s hard to do better than 9/11, so don’t.
V. The Flypaper Theory: It’s easier to attack Americans in the Middle East than in the US.
VI. The He-Kept-Us-Safe Theory: Part of why George Bush will be considered the worst president in United States history.
VII. The Electoral-Cycles Theory: (Non-)Correlation of terrorist attacks with US electoral events.
VIII. The Time-Space Theory: Applying rational-choice theory to terrorism.