Simplest answer- man management

All sorts of army have different ways of keeping people happy in the field. for the British & Commonwealth, the quality of cigarettes was pretty poor, and so general preference was on a steady supply of tea, the rum ration, and the fact that mail received the same priority as rations. for the Russians, it was huge amounts of alcohol, drinking was endemic on the eastern front amongst Russian troops, and it seems to have (alongside rape & the murder of prisoners) the only form of indiscipline allowed. They also made vast use of Tobacco, but preferred to roll their own.

For the Americans, being dry and generally not as addicted to a hot brew as the brits, it was cigarettes. Camel and Lucky Strikes were well made (according to veterans) and serve as a relaxant.