I assume what you mean to ask is why it was defined as a "world war", a total war is something else (usually defined as "military conflict in which the contenders are willing to make any sacrifice in lives and other resources to obtain a complete victory", unlike for example proxy wars or limited wars).

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In either case, it wasn't called World War 1 at the time (the 1 in particular would have been really fucking ominous, don't you think?), it was called the Great War for the most part. It only came to be known as World War 1 after World War 2 when people looked back and said, huh, we did the same thing twice.

See, there is no secret Grand Cabal of International Historians who convene in marbled halls wearing dark robes and decide what we should name certain events (although I now kinda wish there was). Events like these usually get their names from popular perception.

So what does that mean? Well basically it means that no one really decided World War 1 was supposed to be cathogorized as a World War. It was only in retrospect, after having done it again, that people sort of decided that this was a new typ of conflict that existed on a much larger scale than before. There was fighting on virtually ever continent around the world as allies, empires and opportunists all willingly rushed into the fray. Japan took the German colonies. Africa was a mess of Colonial powers fighting it out with a sometimes extremely callous disregard for human cost, British holdings in Asia, ANZAC-countries, the US, Canada and Russia all joined in. This had never really happened before.

Sure, European fighting had sometimes spilled over on colonies or other continents but it was usually quite limited and often mostly naval actions around the world.