Being Vegan doesn't suit everybody, apparently, not even after many years on the diet.
Why would this be?
Different genetic makeup, from racial, climatic, or geographical backgrounds far back in the past?
Logically, Veganism looks like being the best way to feed future human populations, except that masses of food would have to be grown in warmer climates to export to colder climates. Unless the world warmed up considerably, and that, in turn, would bring big adverse farming changes.