In the book of "Night" by Elie Wiesel, in both chapters 6 and 7, why had so many of the prisoners died during the night?

Chap 6: the ss officers made the prisoners run through snow and shot those who were behind.

chap 7: i would say that they died due to the poor sanitation of the cars, they were not fed, and also because in the train carts people would die of the cold that is why very few made it.