Because the NSA is dedicated toward detecting and gaining intel on organized terror collectives abroad. Domestic terrorism is domestic and therefore under the

Because the NSA is dedicated toward detecting and gaining intel on organized terror collectives abroad. 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.