"Candidates had to meet at least one of three criteria: place first through fourth in Iowa, poll 5 percent or higher in one of the last four major New Hampshire surveys, or poll 5 percent or higher in one of the last four major national surveys."
Paul and Giuliani get in because they meet one (or more) of the criteria. Hunter doesn't, so he's out. It's not subjective.
I would not be surprised if the criteria were chosen in some measure so that they would include Paul and not include Hunter, but (1) there's a limit to how many people should be in the debate, and (2) it would seem hard to justify including Giuliani without including Paul, and (3) it would be hard to justify including Hunter and excluding any of the people chosen instead.