A big part of the problem was that until relatively recently, LE treated prostitutes as criminals, no matter how old they were. Johns were charged for soliciting prostitution rather than for child sexual assault, etc.
And then someone in Portland Oregon had one of those brilliant insights that seem completely obvious after the fact: a minor caught in a prostitution sting is a child who has been repeatedly sexually assaulted, not a criminal.
So they stopped treating prostitutes who were minors like criminals in Portland and all of a sudden, these girls started trusting LE and talking to them. And what they had to say curled the hair of even the most hardened officers.
LE in Portland started learning about how minors were trapped into prostitution, how they were moved up and down the west coast, how gangstas and pimps interacted, etc.
For awhile, there was an impression that it was just Portland that had a problem. Because no other city was reporting such things.
But when other cities started following Portland's lead and treating prostitutes like they are victims rather than criminals... well, they found out it wasn't just Portland.
And ever since then, the lid has been slowly blowing off.