I believe it was called a "World War" for the simple reason that the major countries were at war. Most of Europe was ingauged in battle with Hitler. After the fight with Hitler in Europe started so did out fight with Japan. Obviously Pearl Harbor was the cause. But like i said...the main reason was that that whole world did indeed seem to be in war. but not just in war constant war. WWI had happened lass the 40 yrs before.
Both the "Great War" (later called World War I) and World War II are called "World Wars" because there was fighting all over the world.
For example a German "Raider" captured a Norweigen fishing fleet in Ant-Articia. There was a German weather ship captured off the coast of Greenland. The Bismarck sailed in the waters north of Iceland.
There was fighting on every populated continent. Granted the fighting in Australia was mostly by way of Japanese bombers attacking the northern part of that conitinent, but the "Battle of the Coral Sea" was fought to stop Japanese penitriation of shipping routes from the U.S. to Australia.
Fighting took place in Europe, Asia, Africa, South America (the "Battle of the River Plate") and in North America (the Japanese captured some islands belonging to Alaska, and also bombed Oregon, and shelled a part of California from a submarine).
So World War II was truly a WORLD War.
Hope this answers your question. John
Because nearly all the countries in the world were involved in the war.