I think there is this tension whenever a moral norm is changing in society. Vegans are telling people something is worthy of dignity and respect that currently isn’t being granted that status. The cohort of vegans is growing. People don’t like to be made out to be unethical for something they never even considered could be unethical.

this process is how ethics comes to be—a new ideal slowly permeates society as more people adopt it and eventually we decide “cannibalism is unethical” (or whatever) is a general rule and teach that to our children who hopefully perpetuate the rules that ring true for them.

I don’t know for sure, either, but if I had to guess then it has something to do with A. being vegan can be a marker of other progressive beliefs, or perhaps more significantly, B. the consumption of meat is glorified as a traditionally masculine trait