Memes

Well first, I don't think that "is ok to eat meat" is a moral. But it's true that humans haven't tended to find it immoral (though there are exceptions to this in certain cultures, regarding certain meats).

But you make a good point, and I think the answer is that since humans make morals based on their circumstances, and the circumstances of society can and does change, then certain morals become less relevant compared to others. Murder is a fairly constant moral, because regardless of how a society changes, a murderous individual is gonna be bad for it. But on the other hand, there used to be pretty strong morals regarding how dead bodies were treated; you leave them alone. And this used to make sense, since people who messed with dead bodies were likely to get diseases and spread them. But as medicine and science and hygiene improved, this became less relevant as compared to the need to investigate dead bodies to improve understanding of disease and human biology. So our common morals regarding respect for the dead changed.

For veganism, it used to be for most societies that they couldn't afford to simply not eat things, unless they were poisonous. So this need overwhelmed morals of kindness to nature and animals, even though this moral of kindness was still there (respecting nature is a moral found in very many cultures). But in modern day when we now have an abundance of food to the point of large waste, the need to eat whatever you can is no longer as important, and the moral of kindness to animals (and the environment) can be expressed more freely.

And indeed, I think the vast majority of vegans would agree that eating meat is not inherently immoral if there is no other choice, it's only when meat is chosen over other alternatives that it becomes immoral, because it is unnecessary.

Sorry for the wall of text