Keeping the highway open was a deal made with nc and tn when aquiring the lands for the park. It was and is, stupid to bisect it with traffic. The feds could have forced it without the agreement in other ways. Just not popular.
At least they successfully banned commercial traffic, although 18 wheelers are even preferable to the ridiculous loud motorcycle idiots. They dont care about the park, they want to ride their obnoxious toy somewhere fun and curvy.
That road bisecting the park is also the reason there is no entrance fee for GSMNP. It's because the road existed, and the states didn't want the federal government to start charging people for use of this road is what brought about the agreement that allows us to enter the park for free.