Post 9/11 we've had a number of Anthrax attacks.
We've had one kid planting pipe bombs in mailboxes as he drove around the heartland in a smiley-face pattern.
We've had two suspicious jet crashes.
We've had two snipers in the D.C. area.
We've had a tiny blip increase in the number and frequency of train derailments.
We've had a series of cruise ships hit with a bio-agent.
We've had at least two suicide pilots fly small aircraft into bank buildings in Florida (Orlando and Tampa, if I remember correctly).
They've hit a French oil tanker with a suicide raft.
They've fired a couple of missiles at an Israeli passenger aircraft (and suicide-bombed a hotel in Africa).
They've killed scores of Australians in a nightclub bombing in Bali.
We've stopped one suicide bomber from blowing up one of our jets (AKA Richard Reid) with C4 shaped into the sole of his shoe (i.e. the shoe-bomber).
We've caught one bomber trying to set off a radioactive conventional bomb (AKA the "dirty bomber").
It's not that they haven't tried more attacks, it's just that they really don't have the capacity to inflict much widespread harm (fear, sure, but real damage, no).