Answer:
It has been characterized this way because millions of people world-wide died of the "Spanish Flu" during that pandemic. There may have been other epidemics of communicable diseases during other periods of history that were as deadly or more, but due to the lack of frequent and rapid international travel at those times, they were mostly limited in geographical scope, unlike the 1918 flu which was spread world-wide (hence: Pandemic -- "pan" means "all") during WW1.