So here's my theory !! some of you may object this but i feel that there's no harm in imagining :D it goes like this :
It's mentioned in the book that people bet on the tributes. According to me ,the final victor is that tribute who is chosen (bet ) by president snow . You might get confused because of the plot of the movie cuz it shows the perspective of the gamemakers too but in the book it's only katniss.
the only 2 people who were not chosen by president snow were peeta & haymitch .
To teach them a lesson the capitol wipes away their families completely !!!