I rather think it is the self-righteousness and condescension you display in trying to improve the world that people take issue with.
Don't people do the same though with other issues? What I mean is...being self-righteous wrt other causes has been integrated in society - for instance gay rights, or feminism, or racism. This is a cause that's still in its infancy though so it's likely to get more flack and meet more resistance, making the risk for speaking up greater, and the apathy and unwillingness to change more present.
Well, (assumed) moral superiority can be a tough thing to keep to oneself I hear.
For every sanctimonious vegetarian/vegan out there, there's an equally sanctimonious meat eater out there. I know several vegans/vegetarians who were minding their own business, when someone commented on their choices of food during a social event - or who heard through others that that person was vegan/vegetarian and decided to take issue with it and force them to discuss it right then and there - even people who deliberately will make those people eat meat products without their knowledge by mixing things into their food or drink and saying it's vegan/vegetarian, just coz they don't really like the fact that this person is doing something different from them.
People don't like change, they don't like their way of life challenged - on both sides. They need to believe they are right in what they're doing.
Reminds me of a story I heard about permaculture legend Sepp Holzer.
Sepp was doing a q&a with visitors to his farm. One guy asked how to get rid of blackberry bushes in a no-till set-up. Sepp told him to put some electric fencing around it and throw a few pigs in there and two days later you won't have a blackberry bush anymore.
Then a guy raises his hand and says, "But, I'm a vegetarian. What should I do?"
Sepp quickly replies.
"Well then you will have to do the pig's work. Next question."