I believe the biggest things these books have going for them are their heroines. We've seen variations of the plots before. But Katniss and Tris spark something in readers. Here are 5 reasons why:
1. Their Strength
Today's readers are looking for strong heroines who can take care of themselves. Katniss & Tris don't necessarily need someone to take care of them, though that doesn't mean there aren't times when people do.
2. Their Skills
Katniss is a skilled archer who can shoot a bow better than anyone else. Tris learns to fight hand to hand effectively. When we read about these girls, we want to be like them, whether it's realistic or not.
3. They Have Vulnerabilities
An error some authors make in creating their heroines (and heroes for that matter) is to write a perfect person without any weakness. While Katniss and Tris are indeed strong, they aren't perfect. This three-dimensional aspect is what allows us to suspend our disbelief easier when reading their stories. Plus we relate even more since who among us is perfect?
4. Their Hearts Aren't Hard
Most of the evil in these books happens to the girls. They are thrust into situations against their will. If they kill, it is in self-defense. To survive. And both of them face at least one situation that nearly breaks them. After all, they are girls (both sixteen at the start of their books) thrust into a world most women couldn't handle.
5. They Don't Have Time for Bull
Excuse the language, but these are no-nonsense girls (and keep in mind the novels are general market fare). Katniss and Tris say it like it is. They say the things we wish we could say. It gets them in trouble sometimes, but we cheer them on.
If you've read these stories, what about the heroines did you like? Why do you think the novels are so popular?