Commenters above have talked about the signal/noise problem, and they're right, but I don't think anyone has talked about why this problem exists. I have no direct evidence, but I'd bet that after 9/11, there was a high-level conversation in the administration something like this:
"There might be terror cells all over the US, and we might be hit again! Can the NSA watch the electronic communications of all Muslims in the US?"
"Sure, but we can't be sure of knowing who they all are. Besides, it would be considered discrimination to only surveil Muslims. If that ever got out, there would be charges of "racism" and it would complicate things with Saudi Arabia."
"Right, we can't admit that there's a religious war being waged against us. Better to just surveil everyone. Can you do that?"
"Um, sure, but we'll need a really big budget increase."