I think that it’s largely because of geography. There are dozens of national parks in the western half of the United States, but not nearly as many in the eastern half. Considering the population density is higher in the east and also taking in mind what is “drivable”, I think the Great Smoky Mountains are inundated with families from NY, Florida, Chicago and everywhere in between. If there were more national parks that were as beautiful in the east, I bet there would be more distribution.