NSA wiretaps are no good for criminal prosecution. You need to go to a judge and get a warrant. NSA mandate is only national security.
I fully agree all the cops working on drug cases should be on human traffic. I asked a detective I know why this isn't happening. He said drug busts are profitable for law enforcement and they can show the media a big table of drugs and money every week. Human trafficking investigations take much longer and there are no tables of drugs for the media. There is no glory for the police it's much harder work and it's the narcs who get promoted.
I wouldn't want them to violate due process and use their tools to do that anyways, but we already know that they frequently use those tools for prying into matters that aren't national security related. It's no wonder then that they haven't been able to catch more predators, clear they've got some messed up priorities! as you point out