Duke is a national program that isn't deeply rooted with a place. Yes, it's in Durham and in North Carolina but most people across the country do not strongly associate Duke University with those places, it's just Duke University. All these other big time programs are largely defined by the location that is in the name of their university, people outside of those places are not going to be fans of those programs without some kind of tie to that place. Duke probably has far more fans who didn't grow up rooting for their local or state team and therefore adopted Duke as their favorite team as oppossed to adopting a program in which they also would have to adopt that programs state or city.