Why do you think the Hunger Games series is a best seller? What about the series appeals to so many people?

 The Hunger Games series is popular because dystopias are very trendy.  Dystopia is the genre where the community tries to create a perfect world and ends up creating one that infringes on civil liberties.  This definitely describes Panem, where twelve impoverished districts toil away to enrich a gluttonous Capital in punishment for past rebellion.  Some people think that dystopias are so popular because people are unhappy with the world the way it is today, and it is easier to imagine a world that is even worse. 
One of the reasons that this series is popular is that everyone loves an underdog.  Katniss, the no-nonsense heroine, fits the bill.  She is talented, fierce, and unapologetic.  She is also loyal to a fault and volunteers to take her younger sister’s place in the brutal Hunger Games, where each district has to sacrifice two children to fight to the death on live television.
Katniss is a strong female lead, which is a good thing for a book for young adults.  She also comes from a difficult background, which many young people can relate to.  Her father died in a coal mining accident, and her mother is mentally unstable.  When she leaves her mother and younger sister behind, she has to give her mother instructions. 

When I am done with instructions about fuel, and trading, and staying in school, I turn to my mother and grip her arm, hard. “Listen to me. Are you listening to me?” She nods, alarmed by my intensity. She must know what’s coming. “You can’t leave again,” I say. (The Hunger Games, Ch. 2)

Katniss has to be the adult in her household.  That is how bad things have gotten.  Other young adults who have been through difficult situations and have grown up too fast will relate to her story.  Many will admire her and see her as an inspiration. 
Stories of rebellion and civil war are popular, especially when the story is about crooked rulers whose motives are unclear.  The series has many twists and turns, and readers love trilogies.  There seems to be something very satisfying about a trilogy, where three is just the right number of books.