There’s been all sorts of one-off terrorist attacks. The shoe bomber. The underwear bomber. 9-11 even. They tend to go with what works and gets them press. But we’ve countered the things that worked and made them harder. So 9-11 hasn’t happened again... And other attacks have been stopped.
Bombing airliners has been far more frequent than the shoe/underwear stuff.
http://en.wikipedia.org/wiki/Timeline_of_airliner_bombing_attacks