I think there are a few things. Susan Collins started all of this with an idea of a theme, and she wanted to create a series of stories for young adults about the consequences of war. I think that what often happens is it's done in a reverse direction where somebody comes up with a story or somebody comes up with a world and then they try and jam an idea onto that story, as opposed to it being born from a theme.
I also think Susan approached it without pulling any punches, so she's not talking down to the teenagers, she's actually treating them as smart people and treating them as adults, which I think teenagers appreciated, but I also think it's why it helped crossover into the adult world.
On top of that, she created a really rich world, filled it with compelling characters, and told a great story with just a really truly amazing character, Katniss, at the center of it. I think one of the things about Katniss's character is that whether you're a guy or a girl, you can relate to her because she's so real and relatable. She's not a super hero, and she doesn't do the most rogue things. She's flawed and has real and relatable needs and wants and reacts to things. It sort of allows one to put themselves in her shoes, imagine yourself in her situation, and buy into the decisions she's making.More questions on The Hunger Games: Catching Fire (2013 movie):