score:10
What is known:
Buckwheat's yield strongly depends on pollination by insects.
Although it is not universally applicable, in eastern North America, honey bee seems to be the main and the most effective pollinator of buckwheat.
honey bee was not native to Americas, being first bought there from Europe in early 1600s
Sadly, I have no direct historical research to back up the conclusion, and I can only provide it as my guess: buckwheat didn't catch popularity in Americas, because initially it lacked an effective pollinator.