I'm attempting not to have my post here turn into a silly word salad, but a lot of everyone's posts have really got my gears turning on this. All below is my opinion, and is worth exactly what you paid for it.
First, to address the notion of it being inappropriate to drop a 13 and 14 year old into a public nature area together. IMO, and as a parent, this is not at all inappropriate. Half of society decries younger people's dependence on devices, and laments their lack of "experiencing the world around them", here we have two young people that wanted to get out and enjoy a relatively warm day on a school holiday. Perfectly fine. And, what's more, ironically, it was one of these young people's devices that provided the most solid (such as it is) evidence of the perpetrator. These kids did everything right. The person that killed them is solely to blame. Everything else is hindsight, and quite unfair to the families, who are also victims in this. It heartens me to read much of the same from all of you.
Second, I waffle back and forth on what LE knows about the suspect. As much as I want to believe they have narrowed it down to a select few people, I'm more inclined to think they have a solid profile of the suspect. This case has led me down rabbit holes of academic papers, and interviews with criminal profilers, none of which was about this case directly. And from all PUBLICLY available information, what info they have released about the suspect (sketches aside) fits the data of the type of person who commits this type of crime. Their age, gender (which, aside from video and audio is a statistical given), and their social/career circumstances all point to a probability of it being a certain type of person. I think LE is starting from square one, based on eliminating things that would deviate from the statistics. Now they're going back over everything with fresh eyes and a better (albeit, maybe strictly data driven) profile of who committed the crime. To add my own speculation, it would probably be a safe bet for LE to claim that the person was familiar with the immediate area of the crime, probably more so than the average resident. Think about it. If someone has made up their mind to commit an act such as this, and they don't want to get caught, they'd have to be very comfortable in their environment. There's been discussion about organized and disorganized criminals, and I'm going to set the textbook definitions aside for a moment. This crime required a high degree of control over the situation. Wherever it was going to take place, had to be a location that gave the perpetrator a degree of confidence. Even if the act of the crime was a new experience for him, wouldn't he likely want everything else to be as familiar, and comfortable as possible? If he were an outwardly psychopathic mad man (for instance, the disturbed person who killed and injured the hikers on the AT last month), he would have been VERY noticed, before and after, and would have been likely to be quickly caught, either for this crime, or other reasons. This crime took a degree of control (mistakes of allowing himself to be recorded aside)... and to have control, he needed to know his surroundings to get it in, do what he wanted to do, and get out.
Third, as for why, and I hesitate to write this. But I think "the why", is because it was his time to do what he did. It sounds callous, but, Libby and Abby, IMO, were circumstantial to him. If the opportunity didn't present itself then and there, then it would have been later. And who knows, maybe there were other times he waited, and the opportunity never presented itself.
I am not at all an expert. I have ideas that further the idea of the type of person who did this, but I am going to keep them to myself, because they are just guesses, and they don't further the discussion. Again, none of us know all of the facts. It would be safe to say that LE has a lot close to the vest on this for investigative, and privacy purposes. Everything I said, and the conjecture that stemmed from it, is just that, conjecture, based solely on what we know publicly, put through the lens of things I researched, mostly out of curiosity.
On a personal note, thank you websleuths for bringing together a bunch of insightful, respectful, and diligent people. I hope this gets solved. I hope the families get closer to having a little bit of peace. And I hope the community gets a chance to heal once the killer meets with justice.
J.