In no particular order:
1. They are fans of State instead
2. They don't go because UNC Football is not an attractive brand (I disagree with this)
3. Self-fulfilling prophecy.
What do I mean by #3? Well, it's very simply human / group psychology. The permeating atmosphere at a Carolina football game is to sit on your hands, not be rambunctious, and hardly ever cheer. There are plenty of people at any particular Carolina game who WANT to cheer, but they eventually succumb to the status quo of everyone around them, which is to not be loud. You can see this social experiment play out at every single UNC home game:
In a particular section (I'm using the upper deck because that's where my tickets are), there's like, what, 2,000 people or so, in the upper deck? Maybe less. Let's call it 1500. Every game, small pockets of fans -- groups of 2-5 people -- throughout that section, but scattered and dispersed through the various rows, will attempt to be rowdy and get the rest of the section fired up. A few more people will respond to this and also start getting hyped up. Everyone else in the section will ignore them and continue to sit down, not be loud, and not stand up on big plays. Eventually, human/group psychology takes over and the people trying to be cheerleaders ultimately give up and succumb to the norm. This is basic human instinct. People do not generally feel comfortable sticking out in crowds; it makes you feel vulnerable. So, the fans who try to rabble rouse, ultimately get defeated by the status quo. I see it happen every single game.
It happens to me every game. I try to stand up and rouse the fans and get everyone excited, but I start feeling awkward when no one responds, and eventually I'll stop. Only once or twice have I been actually told to sit, but generally, I just sense it. I'll stand, but I'll feel bad because NO ONE behind me will stand, and I know I'm blocking people's view and what not. Eventually, I'll get sheepish and sit down.
Somehow, someway, the culture at Kenan and the Dean Dome needs to change. I don't know how to achieve this.