Because veganism is an emotive subject.
Because vegans are seen as overly 'sensitive' to 'normal' aspects of life therefore need some meaty humour and jokes in their lives to help them 'lighten up'.
Because, in my experience, many meat eaters are very judgey of those who don't eat meat? I suspect subconsciously they are trying to justify eating dead animals and cannot justify it to themselves, so go on the attack to try to justify their life choices.
I'm vegetarian and have had to deal with extremely rude meat eaters over the years. Nothing brings out poor manners more than ordering the vegetarian option. I've had regular piss taking, yes really, I've heard it all before, wasn't funny the first time either. I've had meat eaters sit and interrogate me before, during and after a meal out, and tell me about the pet lamb they had that now fills their freezer. I've frequently had them waft their plate of meat under my nose, or sat tucking into a steak etc looking/staring at me asking if I miss meat...god their manners and judgemental behaviour is appalling.
Am I one of those veggies who ram it under people's noses? NO. I am not a holier than thou teen! The conversation only comes up if I am going to someone's for a meal or I order the veggie option for a meal.
I do know some vegans that are very outspoken and sanctimonious. Meat eaters then tar all vegans, even one's they don't know, with the same brush.
I think what it comes down to is meat eaters not being 100% happy with their own choices in life. Also just being bloody rude.
Can I also add an extra element to this...why do meat eaters rave about meat then at a buffet demolish the veggie option leaving the veggies with only the meat? Happened YET AGAIN yesterday at a party. I was starved as was my eldest who is also a veggie. So lets add, fanatical meat eaters are often hypocrites and rude to boot.