That many vegans with health problems watched those problems evaporate upon return to omnivorous habits does.

Such anecdotes (data more so) are hard to come by due to the verbal abuse ex-vegans tend to suffer.

Again, the plural of anecdote is not data, but I've felt much better since going vegan. I'm willing to grant that may be a placebo effect, but my acne seems to also have improved a bit as well.

Hence room for the correct answer: we are built to consume both plants and animals, but different people respond differently to particular combinations thereof and thus have individual reasons to tailor their diets differently.