It's a tale of brutal violence, calculating strategy, and political evil. It's about a girl.
In posting about the admittedly broad topic of gender and kids' books, I realized there was a lot to contemplate about The Hunger Games and its sequel(s!). From what I've seen, the young and not-so-young adults excited about this series are a pretty equal mix of Janes and Johns, Katnisses and Peetas. Even if you go with the stereotypes of what readers want, there's something for everyone. [Mild spoilers for the first two books follow.] There are weapons, force fields, and mutants that rip throats out, but there's also a young girl willing to suffer and face death to save people she cares about. There are even rivals for Katniss's affection, and those rivals are both pretty worthy. I imagine that choosing between Team Gale and Team Peeta takes some thought.
Still, when readers recommend these novels, I doubt many of them say, "Here's a great book about how characters and relationships develop in a desperate situation!" The books sell primarily on the basis of their main premise and plot. Maybe, shockingly enough, readers of both genders are interested in suspenseful, action-packed stories.