One obvious possibility is that the system that has been erected simply needs fear to be able to continue to exist.
Let some attacks happen (and some Taliban prisons break) from time to time and people can be and will be manipulated via fear, so the powerful stay powerful and the rich become richer. Same old, same old...
Next stop: Middle Class Gone.
Then, people will have nothing to lose again and wake up. Next, we'll have riots and one day, the old system is dead and history will repeat itself once more. So much for the long term.
This shouldn't really come as a surprise, either. Surveillance is not effective for preventing crimes. It is somewhat more useful for investigating crimes after they've happened.
Domestic terrorism is domestic and therefore under the FBI's purview. And despite what Snowden seems to think about mind-reading programs, unless either brother put out on the public Internet or phone communications what they were planning to do, there would have been no way for NSA to pre-emptively detect it even if they were warrantlessly monitoring all domestic communications.
So in a way your question is equivalent to asking why ASLR didn't stop that CSRF attack on the web page you just viewed.
Dzhokhar and Tamerlan were granted legal permanent residence in 2007. Dzhokhar became a nautralized citizen in 2012. Tamerlan's application was on hold due to the DHS being a little suspicious of him.
If the system is working as it should, the NSA could have been prohibited from monitoring the brothers.
Can anyone speak as to why, with the NSA's systems, they were not able to thwart an attack by the ass clowns in Boston? Russia even warned us about them and they made frequent contact with foreigners.
I'm sorry, but that terrorist event seems like low hanging fruit if their system really works to protect us.