Wild poppies flower when other plants in their direct neighbourhood are dead. Their seeds can lie on the ground for years and years, but only when there are no more competing flowers or shrubs in the vicinity (for instance when someone firmly roots up the ground), these seeds will sprout.
There was enough rooted up soil on the battlefield of the Western Front; in fact the whole front consisted of churned up soil. So in May 1915, when McCrae wrote his poem, around him bloodred poppies blossomed like no one had ever seen before.
But in this poem the poppy plays one more role. The poppy is known as a symbol of sleep. The last line We shall not sleep, though poppies grow / In Flanders fields might point to this fact. Some kinds of poppies are used to derive opium from, from which morphine is made. Morphine is one of the strongest painkillers and was often used to put a wounded soldier to sleep. Sometimes medical doctors used it in a higher dose to put the incurable wounded out of their misery.