ND also has a greater share of whites and a smaller share of Native Americans compared to SD.
More of them are also of German descent, and German-Americans are these days one of the most strongly Republican voting blocs. People act like Germans were all completely assimilated and thus there is no distinction between how they and other white Americans vote today, but that's not quite true. It may not be as noticeable as it was back in the day, but German-Americans still have a clear Republican lean and that alone handed Trump the presidency in 2016.
ND and SD are very close in the % of population having German ancestry. ND is considerably more Norwegian than SD, but I doubt that's a reason why it's a bit more Republican.
