Having read through the first thread on this case, StarryStarryNight's recap on her third bullet point is excellent.
To add to everything StarryStarryNight said, I'm going to bring forward a quote from Mpnola in thread 1:
"And if there are 2 girls, if one gets taken or hurt, it is likely the other will, too. They are of the age that they still comply when given a command by an adult. Especially one with threat. And they do not leave their friends behind."
I completely agree with what Mpnola said. At Abby and Libby's age, I would have hesitated long enough out of a natural deference for an adult, even a creepy one, that I would likely have been a victim.
I think it's important to remember how small the window was in which the girls realized they were in danger and had to act. I think there were mere seconds between them thinking BG was creepy and thinking he meant to do them harm.
In the early threads, there is a lot of discussion about the rate at which BG must have approached Abby and Libby, which may be part of what caused them alarm.
Some speculated that BG started trotting toward the girls and that is why he sounded out of breath on the audio.
SeekingJana, in an earlier post, you mentioned how comfortable BG looked in the photos. There was a lot of discussion of that in the earliest thread too. The fact that he had his hands in his pockets and was off to the side rather than being in the middle of the bridge gave many the impression that he had walked that bridge many times before.
Most noted how scary that bridge looked, though most locals had walked it. Websleuthers felt that BG would have his hands out of his pockets and would walk more toward the center of the bridge if he hadn't walked that bridge many times before. The apparent rate of speed with which he approached the girls also indicates his level of comfort on that bridge that is 63 feet above the ground.
Finally, to add to StarryStarryNight's explanation as to why the BG is the perpetrator, there is this early comment:
While LE hasn't specifically stated that BG was the target of the video, several officers have praised "Libby's courage and quick thinking to make the video." To me, that says LE has confirmation that Libby filmed the perpetrator.