In a discussion about Private Clancy of the CEF, who was briefly boosted from private to lance corporal, blogger Medic wrote:
Sometime soldiers were sent on course or permission for a few days or a few weeks, so they needed to be replaced if they were in charge of a group. Also it was not everyone who wanted or liked the promotion. The sergeant had to run from one shell hole to the other, much exposing themselves to enemy fire, many soldiers preferred to stay with the rank of private.
It made me think of this song: