Look on a map that shows mountains, or at least colors National Forests. You'll see that both 28 and 93 follow broad valleys between mountains. 28 is the flattest, straightest way in and out of Salmon.
US93 from Missoula (I90) follows a broad valley, and eventually climbs to a high pass on the state border. It then drops with a few broad sweeps to the Salmon River. At that point the Salmon flows west through wilderness across the middle of the state. 93 follows it upstream as far as Challis. It's 72 and 21 that continue into the heart of the Sawtooth Mtns (around Stanley).