According to the Left, that is because Al Qaeda is not trying.
After all, NOTHING that the Bush Administration does can possibly be right. Therefore, the lack of attacks proves that there was nothing to worry about in the first place.
This ignores the World-wide effort of the USA and civilized governments everywhere (sometimes with USA's prodding) to seek, find, and destroy the terrorists from Indonesia to Portugal. But that is fine with the Left, which ignores ANY positive achievements of the current administration.