Why 'Green' won over 'Citizen Kane'??? A theory
My opinion is that in the late 1930's to early 1940's, some of the winning films like "How Green Was My Valley" won because (in part) they caught the mood, and the fears of Americans who could see the tentacles of war (World War II) coming closer and cloer to ENSNARE them and their family (notably - their young SONS).
Brilliant as "Citizen Kane" was, I suspect the tie breaker for some Oscar votors was - that "How Green Was My Valley" showed outside forces pulling apart a family. And despite some physical separations - the family seemed to stil be one.
Dad, Mom, the kids - especially the young males - even before Pearl Harbor - could strongly suspect that they, too, would soon be drawn in.
(Polls taken by the US Gov't - as seen in some Frank Capra Why We Fight series - show the US change in attitude...)
This is my "historical context" theory.