In some sense we tend to make the most fun of those we like. There are alot of reasons that people poke fun at vegans. For a start, mostly, like with any deviation from the norm, we poke fun at those who are most vocal about it. But also, that population will get characterized by the most extreme of them. Pick anything, drinking, smoking, surfing, religion, whatever, there will be the "true believers" that virtually proselytize about it. But talk to any of them and there are also those who will "explain" to them the error of their ways. Talk to any smoker about the lectures they can get from virtually anyone. Talk to any woman about "mansplaining".
I will acknowledge, the problem that Vegans, and to a great extent vegetarians or anyone with a severe food allergy, are forced to explain or acknowledge their situation because of the social role that food plays in our lives. Otherwise it could be about as well known as what kind of underwear we choose.
I have a weird "quirk", I tend to wear hats alot, even indoors. If I'm in your home, or a church, or whatever and I know you object I'll take it off. But surprisingly alot of people won't. The reason has to do with a time in my life when it was a "thing" that many of us did and that time was important to me. None the less sooner or later someone might ask. Before I launch into a long lecture, I usually just say something to the effect of "because it's important to me". Quite often that shuts the whole thing down. I've offered this advice to many friends who have something in their lives that deviates from the norm. It is amazing how it tends not only to stop conversations, but in fact get people to offer a little bit of respect without a lecture.