They went back to war, because they feared punishment. Even the christmas truce didn’t repeat itself. The officers quickly made sure of that. Remember, this was the first year of the war too.

Ultimately, very rarely does a regular person want to go to war. They want to live a peaceful life, unbothered by anyone, doing their stuff. This truce came about because people were so close, that when they started singing carols, the other side sang the same ones, in their language. That is enough to make you stop and think “wait, those are people in there, same as us”.