This question has puzzled readers since Suzanne Collins's novel was published, and a few theories and answers have been offered, including from the author herself. Why did Cinna choose District 12?
We know that Cinna was a new and relatively unknown designer, and that many of the districts were already claimed by others. So to start with, he had few options. As a designer trying to make a name for himself, he knew that the best way to do so would be to make a huge splash. District 12, virtually ignored in the Games, provided him practically a clean slate on which to create something no one had ever seen before. He could have chosen District 12 as an all-or-nothing gamble on his future success.
This assumes that Cinna was not part of the revolutionary plot before the Games began. If, on the other hand, he had been a subversive from the beginning, he and others likely realized that District 12 — which was lightly policed and highly motivated — was the best location to spark a revolt. It was also the district closest to the supposedly obliterated District 13, which plays an important part it the coming conflagration.
In good literature, though, important parts of a story are never about only one thing. From a literary point of view, having Cinna choose District 12 creates an important dichotomy that changes how both Katniss and the reader view the Capitol.
Cinna and Effie Trinket, Katniss's and the reader's representatives of the Capitol, are polar opposites. Effie accepts her position as escort for District 12 reluctantly, hoping to rise to a "better" district over time; Cinna chooses District 12 intentionally. Effie embraces the over-the-top, superficial fashions of the Capitol, but Cinna favors a simpler, more natural look. And whereas Effie seems completely oblivious to the pain that the Games cause those in the districts, Cinna, when he says to Katniss "How despicable we must seem to you," shows that he recognizes the horror of the Games.
Even though Cinna and Effie are opposites, Katniss and Peeta learn to see them as individuals and ultimately to like them both, which, because they represent the Capitol, humanizes the people of the Capitol. The humanity and individuality of these two characters show that the people of the Capitol are not uniformly evil and repressive, but are just as various and dissimilar as the people of the districts.
Thus is highlighted one of the central tenets of dystopian fiction: The conflict is not between one individual and another; it's between the individual and the system-the passive acceptance of repression, the status quo. Katniss isn't in a fight against the people of the Capitol but against the system that the Capitol maintains.