WWI was not mainly a war in Europe, but its easy to assume this as all the most accessible media on the subject is largely about the horrors of the Western European Front. WWI was a global conflict between global empires, and thus justly deserves the title of 'World War.' We see major actions and campaigns in the Far East, Central and West Africa, the Levant, the Ukraine, global and unrestricted sea warfare, and so on and so forth.

The Napoleonic wars had actions outside of Europe, to be sure, but to call them major would be dubious, at best. At this time period, we do see some minor conflicts in the Caribbeans, such as Napoleon's attempt to re-instate slavery in Haiti, but the campaign was destroyed by Yellow Fever before it could really develop. Most other French possessions were either cut off from the homeland or seized without nary a shot by the roving British Navy after 1805. We can discount the abortive attempts to reach India in 1798 through Egypt, despite the importance they play in the mythos of Napoleon I. Now, we also have the wars of 1812 and the Indian rebellions, but these have but the loosest of connections to the Napoleonic wars; in the former case the US was ostensibly acting out against the highly questionable 'right of search' the British conducted on foreign and neutral ships, and in the latter the rebellion leaders were given nothing but sweet words by the French government. Such peripheral actions had little strategic significance compared to say...the Palestinian campaign in WWI (goal: Knock the Ottomans out of the war) or the Eastern Front of WWI. By comparison, these peripheral actions between 1801-1815 had debatable or outright negligent effect's on the European home front. The Napoleonic Wars were won and lost in Europe.

It could be said that the first 'global war' was in fact the Seven Year's conflict; and it's much in the vein of World War One in terms of strategically significant moves abroad in the Americas and East Indies. Even this, however, can be discounted by the overly euro-centric combatants. Unlike in WWI, all the major players were in Southern or Western Europe.

Both the Napoleonic war and what we now call "WWI" were, in their immediate aftermaths, called "The Great War". The conflict of 1914-1918 was only relabeled when there was another war, which was even more world-wide; and 1939-45 was pretty clearly continuous with 1914-18, so it made sense to give them both labels from the same theme, so to speak. By then the Napoleonic wars were too far in the past to be included in the same naming scheme.