A) He clutzed the bomb just like they said, after all he did go to the University of Bridgeport which is not exactly known for rocket surgery
B) The bomb was more sophisticated (more powerful, radiological, etc) than we've been told and therefore was more complex to detonate
C) Shahzad is a US agent and will assist, is, was, or will become mole bait for infiltrating the Taliban and therefore the bomb was not supposed to injury anyone
D) Shahzad is a US agent and will solidify public opinion to continue bombing the Taliban all the back into the 6th century where they can visit with Muhammed directly for guidance