Governments bombed the civilian populations of their enemies because

Because they considered civilian populations as more than "civilians," since they were directly aiding the war effort by industrial production. They also wanted to demoralize civilian populations so that they would not continue to support their government's effort to keep fighting the war.

Explanation:

In World War II (which I think is the context for your question), both the Allied and Axis powers targeted civilian populations. The Germans, for instance, tried to break the will of the British people with sustained air raid bombings. Later in the war, the Allies conducted fire bombings of whole cities in Germany and Japan, as well as the catastrophic atomic bombing of Hiroshima and Nagasaki.