I'm honestly not sure. I only knew those 4 people who went Vegan & the were on my animal care course at college.
At a guess, I'd say (just theory), because the nutritional requirements for people can vary. Many factors can influence this such as blood type, ancestors diets etc. Some people do well on higher protein, others on higher vitamins. Google & read for yourself, I just took a quick peak (very quick, so please don't challenge me on anything). I think things like your metabolism & thyroid also have an impact. My blood is O negative, so according to the web page I just visited I do indeed do better on a high protein & Lysine diet (as I already knew) as opposed to eating vegetables.
I just seem to really struggle digesting fruit & vegetables
& it is painful! Cereal is another thing I cannot eat.
Nor do I get cravings to eat those foods. My DH is another story, that guy practically lives off of boxes of cereal which would probably kill my digestion! Also he seems to crave fish a lot.
Say for instance if your family have eaten fish / seafood for generations your body will be pretty well adapted to draw all the nutritional value from that food.
Likewise if your family have lived off a rice / grain / vegetable diet for generations then they too will have adapted to lower levels of certain vitamins etc.
Its all about evolution, what your ancestors ate more of in their area where they lived will be what you thrive off the most because of genetics. Whether its a meat, veg or grain source doesn't matter.
Hopefully in a few more years we will learn more from science.
And I have just realized my diet is almost identical to my dogs. Lean meat, organ meat & a bit of vegetables on the side. My dogs are on homecooked meals btw
we all eat pretty much the same stuff. The only thing different is they gnaw on bones daily, I only get to gnaw on a bone 3 times a week!
Also, when you feed the dogs cereal, the same thing happens to them as what happens to me... they get ill with indigestion. They can't stomach it anymore than I can. No more than a handful.
Some of us just can't stomach lots of grains / veg.