This is just from my personal experience, but as someone who is very sympathetic to the moral/ethical arguments from vegans but is not vegan themselves (though did try it out for a bit, lost a dangerous amount of weight and developed an intolerance to potatoes which further reduces available food to absurd levels sine potatoes are in tons of vegan food), is that it's the minority of vocal/aggressive vegans that have largely (though certainly not entirely) caused the poor reception.
My reasoning: People don't like to be judged (at all, but especially when the data behind the judgment is not so clear cut correct), and even just being in the presence of vegans can make some people feel judged. Even if the vegan says nothing, people can feel like they're being judged (to be clear, this is not the vegan's fault!). Now, add a small but vocal and aggressive minority of vegans who will explicitly judge/criticize, and you have a recipe for contention. It shouldn't be this way IMHO, but it's human nature.
My reasoning: People don't like to be judged (at all, but especially when the data behind the judgment is not so clear cut correct), and even just being in the presence of vegans can make some people feel judged. Even if the vegan says nothing, people can feel like they're being judged (to be clear, this is not the vegan's fault!). Now, add a small but vocal and aggressive minority of vegans who will explicitly judge/criticize, and you have a recipe for contention. It shouldn't be this way IMHO, but it's human nature.