>The authors note that the consumption of grain led to poorer health -- and
>probably killed some outright -- but something was pretty compelling about
>grains to make people abandoned a WOE that had served them well for millenia
>and adopt a lifestyle that was so focussed  on one food.

Snake oil salesmen, or whatever they would be called then.  Someone
probably told them that it would cure all their ills and then they got
addicted.