Scientists have found that Japanese people seem to possess gut flora for digesting seaweed that Americans do not. The sample size for the bacteria collection is fairly small though. They theorize that the Japansese might have acquired the bacteria through eating raw seaweed. An interesting implication they make is that this bacteria not only affects the digestion of seaweed, but carbohydrates in general.
I wonder if it's more a reflection of the homogenized American culture we live in then anything. Plenty of my recent ancestors consumed seaweed as well in both Scotland and Wales- mostly laver, but they also used seaweed as livestock feed and fertilizer. Coming to America (by force or choice), they lost this tradition. Perhaps the loss of a tradition is more than just a loss of cultural knowledge, but an loss of a species inside us as well. When we are reviving traditions these days, it's often in the context of a sterile food system that might not allow us to truly regain what we lost.