I'm not totally sure if this is entirely what you mean, but I assume that the distinction between a "World War" on the one hand and a purely European war on the other is based on America's entry into the conflict in 1917. You might find the answer to your question here, which explains what factors led the US to enter the conflict.

The Napoleonic Wars were Europe-wide at many points, but the United States stayed aloof from those conflicts as far as sending ground troops to fight within Europe is concerned.

If we want to argue semantics it could be argued that it's because the term "world war" didn't exist according to the English Oxford Dictionary until just a few years before the First World War. Its definition was a war fought by important or major powers (it didn't actually say a war fought all over the world). Wars before that may have fit the definition but as said the term did not exist at the time.