Why is flanders field used so often on Remembrance Day?

I think mostly because a Canadian wrote the poem "Flanders Fields".

Flanders was allegedly a generic name for battlefields in the county of Flanders in Belgium. Canada fought many of their most important battles there; Ypres, The Somme and Passchendaele.

John MacCrae wrote the poem during the battle of the Somme, in Flanders.