Shortly before the 68th hunger games, a small revolution was born and then quickly put out but it struck fear within Snow. Once most of the revolutionaries were taken care of he had to scare the general public by creating the games directly following. It was almost like a quarter quell, as double the amount of tributes are selected from each district. Given as such, the remaining two tributes, from any two districts can choose to kill each other and have only one victor. Or choose to let the other live and have two crowned victors.
To remind his citizens of what happens when you test his patience, he made the arena an almost life like replica to what the Capital depicts the 13th district as, a lost city destroyed by nuclear war. The arena is split in between the city and a radiation forest where.. radiation definitely has effected the wild life. Poison flowers, mutated overly aggressive deer and acid rain. The tributes are split up 24 in the forest and 24 in the city and then they can travel to whichever side they deem safer.