I-90 in western SD isn't the busiest highway, but it does see a fair amount of truck traffic. Also, besides Covid, much of the vehicle traffic is tourism, and December isn't the height of the tourist season.
Other factors are that Wall is the western entrance to Badlands National Park, so a lot of tourists go through the park, rather than taking the Interstate east of Wall. The east entrance connects to I-90 further to the east. And, that early in the morning, a lot of the tourists, even in summer, wouldn't be at Wall. They have few motel rooms, and it's a long ways to the towns that do. By the time they eat breakfast, and drive an hour or more, they won't be there until later in the morning.