Courtesy of the NEFA Foundation:
"Why Have We Not Been Attacked Again? Competing and Complementary Hypotheses for Homeland Attack Frequency", June 2008.
The Defense Threat Reduction Agency and Science International Applications Corporation released a report titled, "Why Have We Not Been Attacked Again? Competing and Complementary Hypotheses for Homeland Attack Frequency." As the authors explain, "The hypotheses analyzed in this report can be divided roughly into two broad categories. The first category – Capabilities – suggests that terrorists have been unable to succeed in conducting another large-scale attack on the homeland due to the effectiveness of U.S. defenses or because of the terrorists’ limited capabilities. The second category – Motivations – assumes that a number of terrorist groups possess the ability to attack the United States but have chosen not to do so for a variety of reasons." There are more than two dozen competing explanations and "this study suggests that identifying a complete and consistent explanation for the non-occurrence of a subsequent attack on the U.S. homeland may not be possible."
208 page PDF

Anybody have a favored hypotheses or combination of?

I have a combination that may seem contradictory:
Hypothesis F) Terrorist threat has been massively exaggerated
Hypothesis H) Al-Qaeda is waiting to acquire a CBRN capability
Hypothesis L) Al-Qaeda’s next attack must surpass 9/11
Hypothesis Z) Al-Qaeda’s goal is to “bleed” the United States dry economically