Motive is the biggest puzzle. If LE really has a lot more audio, wouldn’t the “why” of BG going after the girls have been recognized? And if not, wouldn’t LE have just said something like, to the best of our knowledge, the BG didn’t seem to have a reason?
What harm to the investigation would come from LE giving that clue? I don’t recall them ever saying for sure that the girls did not know who BG was, or at least recognize him in some way.
If someone was marching you down a hill, even with a gun, wouldn’t you ask why? Or, did the girls already know they were heading somewhere with BG, and perhaps at that point still not realize it was about to go terribly wrong? Maybe to find a scavenger hunt item, look for something dropped, even to secretly go try their first joint or a can of beer? I know, I know, they were good kids, but... they were also young teens exploring independence.