How Come There Haven't Been Any Significant Terror Attacks on the US since 9/11 and the Anthrax Attacks?

A few basic possibilities:

1) Law enforcement has cracked down on terorism to prevent new attacks

2) Bush's war on terrorism has scared away the terrorists from attacking us again

3) 9/11 and the anthrax attacks were fluke attacks that were just very lucky and could never be repeated

4) 9/11 and the anthrax attacks served a horrible political purpose, and repeating them would be detrimental to the people who have benefited from the attacks.

Analysis:
1) Since there has been lack of prosecution against 9/11 collaborators, an inability to catch the anthrax mailer and there has been no announcement that major attacks have been stopped, we can fairly assume this is not the case.

2) there is certainly no reason to think this is true.

3) while the 9/11 attacks may have just been incredibly lucky, there is no reason to think that is the case for the anthrax mailer. Moreover, simple luck cannot explain why new attacks have not been stopped (and surely they would have made a big splash if a major attack WAS stopped).

4) This seems to me to be the most likely explanation. Thus, there will not be any new attacks unless those responsible for the 9/11 attacks or the anthrax attacks have a political need for them.