Harsh weather, low morale from lengthened contracts, low on supplies, poor health of soldiers.
Firstly, many soldiers had been previously wounded in battle, if not dead, from the battles preceding the winter at Valley Forge. The majority of the Continental Army was left in poor health whether it be from disease, malnutrition, or starvation. This was partly due to the lack of supplies that the Continental Army possessed (very few even had shoes). On top of that, soldiers had to deal with the extreme temperatures that winter brought upon them.
