Pretty young girls like Abigail Hernandez are kidnapped so that they can be sexually exploited. That is what happened to Elizabeth Smart, Jaycee Dugard, Elizabeth Shouf, Shasta Groene, Tara Burke, Shawn Hornbeck, and Steven Staynor (all of whom lived to tell their stories). It was also the motivation behind the kidnappings of the vast majority of kids, including my Polly, who were ultimately murdered as a result of their abduction.
I suspect that she got away by earning her kidnappers trust and securing enough freedom to affect her escape, or he was distracted and she took advantage of an opportunity, or some combination of the two. It is also possible that she convinced him to let her go, although knowing the mind set of desperate sexual predators, I find that unlikely.
The important lesson here is that she managed to stay alive long enough to escape Crazy Nate Kibby, a dangerous sexual predator.