Spanish flu was so deadly because it was basically the perfect time for a pandemic to strike. Lots of people moving around the world, poor hygiene, poor general health because off the war, lack of resources because of the war, lack of communication about the spread of the pandemic because of the war. Basically, without WWI, Spanish flu would have been a minor event.
Fun fact which I learned from a WWI podcast, Spanish flu actually originated in the USA and was transported to Europe by US soldiers. It was called Spanish flu, because as a neutral country, Spain didn't have the same restrictions on the press as the combatant countries and that meant it was the first place which actually reported it, even though it had been rampaging across France for a while.