Because, at the time, it was the greatest, worst war the world could remember, and most Europeans, anyway, did not wish to see anything like it ever again.
Because it involved most countries in Europe and these countries ruled most of the world at the time. It was the largest war in history at the time by far.
Because testosterone-crazed men decided it was the most significant human accomplishment to date.
Well, they would have called it the Just Okay War had they known what was around the corner. It's true, the old, once testosterone-crazed men told me.
LOLOLOL! OMG, is that funny!
"The War To End All Wars"