Because nobody likes self aggrandizement. The perception that so many people only do it to make themselves appear to be better people because of their morally superior choice is often a vile taste to anyone who hasn’t made that same choice.
We all know eating meat is bad and for the many reasons for it. What we don’t want to hear is that someone made the switch and that their bleeding heart simply couldnt take it anymore.
I eat beyond meat and I do my best to transition, yet, I’d never say that for the purpose of making myself seem like a better person. Vegans typically do.