It all depends on your historical perspective. As late as Shakespeare's time, putting a city to the sword for forcing one go through the trouble of taking it was acceptable. Genocide as the natural outcome of losing a war goes back to prehistory. Check out the Bible. In fact, there are those who think we h.sapiens are responsible for the deliberate extinction of h.neanderthalis.
Therefore, the real question is not why the distinction between civilians and combatants became so blurred in the second half of the 20th Century. The question should be how and why did Western Civilization manage to maintain this distinction for about 500 years.