Daniel Byman, Director of Georgetown's Security Studies Program and the Center for Peace and Security Studies, blogs on the subject at the http://blogs.law.harvard.edu/mesh/2008/11/surprise_no_october_surprise/">Middle East Strategy at Harvard blog. Some possible explanations (read the whole post for more):

• Bin Laden has other fish to fry. Although Americans understandably focus on the threat Al Qaeda poses to the United States, from Bin Laden’s point of view we are only one concern of many—even if we still are a favorite target of his rhetoric. Al Qaeda’s primary day-to-day focus now is on events in Pakistan, where the organization is based, and Afghanistan, where it is helping support the massive insurgency that is battling the U.S.-backed Karzai government. As if this were not enough, Al Qaeda has ambitions in Iraq, the Maghreb, and Central Asia as well as against Israel. These theaters are important to Al Qaeda leaders, and many in the organization would prioritize them over attacks in the United States. Even if the United States remains the primary focus of the leaders of the Al Qaeda core, expanding operations in several of these theaters gives Al Qaeda opportunities to strike at America outside the U.S. homeland. Iraq and Afghanistan allows it to showcase one of its preferred methods: support for insurgents.
• Al Qaeda’s operational capacity is limited. Al Qaeda has reestablished a base in tribal parts of Pakistan, and its operational capacity is growing when compared to the organization’s dark days in 2002. Yet while Pakistan is an excellent haven, in many respects it is a tougher one than the Taliban’s Afghanistan. From Pakistan Al Qaeda can still plot attacks, and its propaganda is prodigious. However, its leaders must also spend much of their time battling or bribing government forces, hiding from U.S. Predator strikes, or otherwise focusing on their daily survival.
• U.S. government efforts at home are paying off. The Department of Homeland Security is much-maligned, but at least it is trying to stop jihadists from entering the country. And trying counts. The FBI has made numerous arrests on terrorism charges (often, we find out later, on quite thin grounds), suggesting that it is aggressive in going after any potential jihadist threat at home.
• Aggressive intelligence efforts abroad keep us safer at home. More important than strictly domestic efforts, U.S. intelligence is working with its counterparts around the world to disrupt the organization, making it harder for Al Qaeda to do sustained operations. Remember, the 9/11 attack involved not only the United States and Afghanistan, but also Germany, Saudi Arabia, Malaysia, and other countries. Such a global plot would be far more difficult to orchestrate today. Senior leaders would be more likely to be killed, and junior operatives would be more likely to be arrested.
• Al Qaeda wants to outdo 9/11. Bin Laden does not think small, and he consistently seeks terrorism “spectaculars” against the United States (for example, the plot to bomb transatlantic flights from the United Kingdom, which was foiled in the summer of 2006). A spectacular attack might inflict mass casualties like 9/11, or it might involve a lower casualty but novel method, such as chemical weapons. This ambition may dissuade Bin Laden from a low-level strike before the election, as he wants to save his powder for a time when he can inflict the maximum damage.
• There is no “Al Qaeda of the United States.” Even if the United States were not more aggressive at home and abroad, Al Qaeda’s ability to operate in the United States is limited. In contrast to Britain, Egypt, France, Saudi Arabia, Spain and many other countries, the United States does not have a significant domestic jihadist network within its borders. Government prosecution efforts reveal that many arrested plotters were incompetent dreamers who had little or no ties to the Al Qaeda core, in contrast to their counterparts in Europe and the Arab world. Infiltrators Bin Laden sends to the United States would find it hard to gain local assistance as they prepare for an attack. The few radicalized American Muslims might still attack in Al Qaeda’s name, but the likelihood is far lower than in many other countries, and the skill level of the attackers would probably be limited, making a 9/11-scale operation particularly unlikely, which (as noted above) is probably one of Bin Laden’s goals for operations in the United States.