Why haven't we been attacked again yet?

This is a good article, which proposes several reason why we haven't endured another terrorist attack on US soil. And none are the standard ones offered up by the government. Here are a couple good excerpts

Instead, Americans are told -- often by the same people who had once predicted imminent attacks -- that the absence of international terrorist strikes in the United States is owed to the protective measures so hastily and expensively put in place after 9/11. But there is a problem with this argument. True, there have been no terrorist incidents in the United States in the last five years. But nor were there any in the five years before the 9/11 attacks, at a time when the United States was doing much less to protect itself. It would take only one or two guys with a gun or an explosive to terrorize vast numbers of people, as the sniper attacks around Washington, D.C., demonstrated in 2002.

...

It may well have become more difficult for terrorists to get into the country, but, as thousands demonstrate each day, it is far from impossible. Immigration procedures have been substantially tightened (at considerable cost), and suspicious U.S. border guards have turned away a few likely bad apples. But visitors and immigrants continue to flood the country. There are over 300 million legal entries by foreigners each year, and illegal crossings number between 1,000 and 4,000 a day -- to say nothing of the generous quantities of forbidden substances that the government has been unable to intercept or even detect despite decades of a strenuous and well-funded "war on drugs."

...

It is also sometimes suggested that the terrorists are now too busy killing Americans and others in Iraq to devote the time, manpower, or energy necessary to pull off similar deeds in the United States. But terrorists with al Qaeda sympathies or sensibilities have managed to carry out attacks in Egypt, Jordan, Morocco, Saudi Arabia, Spain, Turkey, the United Kingdom, and elsewhere in the past three years; not every single potential bomb thrower has joined the fray in Iraq.