I think if there isn’t a victor for every district, they’d just do “Every living victor competes”, the point is to make a show of it to kill Katniss so it seems “legitimate” since they can’t just directly execute the victors, especially not Katniss since she has popularity. They have to keep up the pretenses of these “games” to maintain legitimacy.

But mostly its just storywriting… they want to make it “perfectly balanced” with the “two from each district” theme to make the story fit better.