there are almost certainly tactical reasons as well
early in a conflict, troops, Generals, Presidents and Kings, all tend to expect and want a quick and decisive victory...
That causes troops to overextend themselves and take greater risks...Which leads to relatively high casualty rates.
As the conflict "settles" into the "routine", then more time can be taken in the tactical decision making process, events are not as fluid and the opportunity for decisive "runs to victory" are much fewer, resulting in relatively fewer casualties.