Presumably because LFL’s archives are so large it takes a long time to go through them.
Also, they might not have known what was stored in there. The archivists probably had that box way down on a “to investigate sometime” list, but Edwards recognized it for what it was and brought it to light.
I’m reminded of how one time Ralph McQuarrie found a bunch of his old art from SW tucked behind the furnace in his house.