Why do historians refer to WWI at a "world war"?

It is because the system of entangling alliances caused much of the world to enter the war over the nationalistic Serbian spark. Also you have countries not directly involved in combat sending supplies and etc to help a certain side win. Globally much of the industrial, and some not (imperial footholds) were involved in this war. Before we had never really had this happen on such a scale. This is why it is referred to as the "First Great War."