Why are poppies World War 1 symbols?

Poppies, small red wild flowers that have knocked out Dorothy in the Wizard of Oz like a surgical anaesthetic are somehow the symbol for World War 1 remembrance but why? 8.5 million Soldiers died in World War 1 (called the Great War back then as they didn't expect a failed artist from Austria to get his revenge through politics) in battlefields that in the spring would sprout bright red flowers in the spring. Our story begins in 1915 just after the second battle of Ypres (so called as it was near the Belgian town of Ypres) when an Allied artillery unit surgeon from Canada named Lieutenant Colonel John McCrae spotted a cluster of poppies. Inspired by the poppies, McCrae wrote a poem entitled In Flanders Field (Flanders is in the north of Belgium) in which he channeled the voices of the fallen soldiers whose remains were under the roots of the poppies. The poem was then published in Punch magazine later that year and it became very popular at memorial ceremonies for the dead. Two days before the armistice in 1918, an American named Moina Michael read the poem in the pages of Ladies' Home Journal and decided in honour of the fallen of Flanders field to wear a poppy badge which led to her campaigning for the flower to be made a symbol for World War 1 remembrance and by 1921 the first poppy appeal was held in Britain.
Poppies wreaths are laid on war memorials and worn as badges in Britain on Remembrance Sunday (the second Sunday in November) and Armistice Day while they are worn as badges in the United States on Memorial Day (on the last Sunday in May) to commemorate fallen soldiers of World War 1.