Maybe because they're tired of vegans trying to shame them?
More flies with honey than vinegar. It is the only way to get beyond 3%.
But clearly, vinegar seems to be preferred. So nothing will change.
Thus not worth discussing further.
It's not vinegar or shaming; it's presenting actual information (when asked), and I think some, not all, vegetarians are as defensive as meat-eaters when the facts and horrors of the dairy and egg industries are presented to them, because they don't want to hear it, just as meat-eaters don't want to hear it.
Years ago, a friend who was vegetarian was quite blunt, in a polite way, though while I was eating chicken, about informing me that I would never eat chicken again if I knew what happened to chickens in the slaughterhouses. So after dinner, I asked her, and she told me. That's pretty much when I went vegetarian. It took me years to eliminate dairy and eggs, even after I learned about those horrors because I didn't think I had the willpower to do it. So really, it was about the convenience of my life, not of the animals' lives. After a few years, I felt increasingly guilty about consuming eggs and dairy, and one day, I just stopped. I am truly grateful that my friend helped send me on my veggie journey. She, too, is now a vegan, after I returned the favor of letting her know about the evils of dairy and egg production.
I do agree that every little bit helps, and I try not to judge, because people have to start somewhere, just as I did. But if someone goes on about "happy meat," I cannot stay silent. Killing is killing, and to me, that is pretty black and white. The debate about "happy eggs" is a little different, if people are truly consuming eggs from local people who don't cage or crowd their chickens into small spaces. The argument is more about commodification than cruelty, and some people will never let go of the idea that animals are here to serve humans. I have a friend who finally has stopped crowing about "happy meat" around me, because my answer is always: You do realize that it's not so happy for the cow whose life is cut short, right? She pats herself on the back for consuming it, and I just want to scream most of the time. It takes every fiber of my being not to be a total b*tch about it, because I no longer can understand how people can consume what was once a sentient being.
The whole "happy meat" campaign does seem to me counterproductive to actually ending meat consumption, because it does somehow lend justification to, and even encourages, continued consumption. But, as Tom said, I don't want factory farming to continue with the horrid practices they use on animals.
What it comes down to is convenience, and I think a lot of vegetarians aren't willing to make further sacrifices when it comes to dairy and eggs. Many just think it's too hard to be vegan, both for them individually and socially. Just the other day, a friend and I went to lunch. She picked the place, but she always makes it known that it's a bother to "find" a place where I can eat. I always tell her that it shouldn't be a bother, as I can eat anywhere. So when she said chose a place that had a veggie burger, I was like OK, fine. I then asked the waiter whether the burger had egg or cheese, and when he returned and told me that it indeed did have egg, I said thank you and promptly ordered a salad and fries. My friend had the nerve to say, "Would it kill you to eat a little egg?" I'm sure the look on my face said it all. I was livid; I wanted to throttle her. I had to take a deep breath, and then I said, very directly and with some annoyance: "Yeah, it would. It's against my principles, as you know. I don't consume anything from animals because I don't believe they are here for our pleasure." She shut up quite quickly and moved on to other topics.
So, yes, being vegan can present difficulty in social situations, but for me, it's a no-brainer to live in a manner that eases the exploitation of other beings. Am I perfect? No, no one is, but I will not compromise with "just a little egg" so that I can make someone else feel less guilty about chowing down on an egg and cheese omelet or a beef burger. I don't offer my take unless it comes up in conversation, but I have warned more than a few friends that if they want to debate me on this, I will be blunt; I will not sugarcoat it. Thus, the frustration on the vegan side is that vegans can't understand how vegetarians who are in full knowledge of what happens in the egg and dairy industries can continue consuming those products, given what they now know. That makes it difficult to work together sometimes when it comes to outreach. I try to remember that I was once in their shoes, and that it took me a while to come around. That gives me patience to understand where they are coming from. And I figure if I could do it, someone else might be able to as well. I try to live by example and educate when called upon.
I think FortyTwo is right that it could be centuries before animals are not considered food, but then again, I always have hope that things will move a bit faster if we just keep at it. Every person who reduces consumption of animals and their byproducts is doing something positive in my eyes. As someone said eons ago, "Veganism is a journey, not a destination."