Because, deep down, many people don’t feel good about eating meat, so they try to disparage veganism to make themselves feel better.
Or:
Most people are making their own choices and take a live and let live approach and don't take kindly to anyone (vegan or otherwise) trying to take the high ground on one issue every five minutes.
Eg. One of my friends from university loved talking about environmentally friendly they were for being vegan, why everyone should be vegan etc. My then DP got fed up with hearing about it that he did a carbon footprint calculator with her. It turned out our carbon footprint was lower than hers and her DP because if the house we lived in, heating set lower, only running one car, holiday choices, not having pets etc.
DP and I wouldn't have cared either way had we not been subjected to "our way is so much more moral than yours".
Even in your reply there, it couldn't be that people get irritated at the conduct of some vegans, no it has to be that people feel awful about eating meat and so like to have a go at vegans because the righteous vegans show everyone else how immoral they are. It's that attitude that pisses people off.
BeingATwatItsABingThing
I think it's good for everyone to reduce their meat consumption. I also think it's much better for everyone to make smaller choices than doing all or nothing.
What I can't stand is people who get on their high horse about one of their choices being so much more ethical, and act like anyone who doesn't make the same choice as them is somehow unethical, destroying the environment etc.
Eg. I've got an acquaintance who is now using washable nappies (which is great we do a mix to make small changes for the environment). But they now act like they couldn't possibly go back to using disposable ones now they know how awful it is. At times it sounds quite sanctimonious and I know for a fact they drive everywhere and take lots of flights so I can't help but roll my eyes when they get started. There's no need for the self-righteous approach. Just make the small choices that are right for you like everyone else does and save the lecturing.