I think some people just want to make something their complete identity and to feel morally superior. I get that people get passionate about some things, but being condescending towards others is not the way to win allies.

I did a veggo stint years ago to test it out for myself (and some health reasons). Went over to a relatives for a bbq and only ate salad, rice and other sides. Didn't complain, just went about my day.

The host noticed and made a massive thing out of it (there were like 20 people, so he wasn't cooking just for us) and kept pestering my why I'm going vegetarian. I was replying politely that it's a personal choice etc....he kept pushing, so finally I said I'm only vegetarian at his place, cause he cant grill for shit.

No more questions after that.