This is all in general terms, and may not apply to this case at all.
But first, I don't think a victim can be forced to cooperate. It's also possible that a victim does cooperate but the suspect/s can't be identified/located or is deceased, or LE doesn't think they have enough proof for charges to stick. And even with unsolved murders and currently missing, endangered minors, when there are no more leads for LE to go on at the time, they have to stop.
It's also possible that if a minor found home life unbearable or wanted to leave for a love interest or whatever, that they would not view whoever assisted them as a criminal but rather sees them as a friend/s or past love interest and would not want to turn them in. They may see it that way just at their current young age, or forever. Just saying, legalities and how people feel about situations or people in their life are not always the same.
Just one example here, say a fifteen-year-old had decided to run off with her eighteen-year-old boyfriend, then she moved on to some other guy when she was eighteen. Say she's now 19 or 20 and doesn't want to tell on her original boyfriend. Even though I doubt many people would think that or similar scenarios were okay, I could see why LE might decide to just drop it. MOO