The question is, why was that anthrax attack a one-time bad deal? We responded to 9/11 by overthrowing the Taliban in Afghanistan and (IMHO) we responded to the anthrax attacks by invading Iraq. Why was it possible to attack us once but not again?
(1) The personnel trained to carry out the attack died in the airplanes on 9/11
(2) It didn't start on time ~ like they'd thought. The attackers imagined hundreds of thousands of people throughout the country coming down with pulminary anthrax shortly after the WTC and DC (Pentagon) attacks. Obviously it's not the sort of thing that creates very much of a dramatic flourish.
(3) Although there was widespread exposure to anthrax, only a small fraction of those exposed got sick, and even fewer died. Unexpectedly (to everyone) Cipro "cures" the condition if you catch it in time.
(4) If there's been another attack using the USPS as a vehicle, you haven't been told about it, and the anthrax (and lebenteenzillion other germs and poisons) were detected in a timely matter by special equipment installed in postal facilities since 9/11.