US95 starts on the Snake River, and crosses mountains and uplands, while the Snake flows through Hells Canyon. Along the way, 95 travels along the lower Salmon River, a major tributary. Lewiston is the lowest point in Idaho.
So the grades are mainly the result of passing in and out of those river valleys.
The drop from New Meadows to Riggins on the Salmon is relatively long and gentle. The climb at White Bird to the Grangeville Plateau is more noticeable. Another long drop to Lewiston, and again the climb north out of Lewiston is significant.
Google Maps with terrain mode gives a good idea of where you are crossing in and out of these river valleys. Directions in Bicycle mode show an elevation profile.
The only alternative is I84 well to the north west, and US12 back east. That has freeway grades, but actually climbs higher.
At Whitebird there's an old grade, that is longer, gentler and twistier, designed for old cars with a lower power to weight ratio. It passes a historic battle field. There is also an old grade north of Lewiston.