From my experience as an ex-vegan I think part of the perceived sense of humour failure is that vegans have heard it all before. A lot of people think it's fine to question a vegan diet & you go get tired of it. If you heard the funniest joke in the world eventually you would get bored of it & it would cease to be funny.
There's even 'defensive omnivore bingo' which predicts the top things omnivores are likely to say & every person on a vegan forum I frequented has heard them all, many times.
Which is why some vegans might want to get together to discuss food without being pestered.
Genuinely curious people, fair enough, but this is just having a dig for the sake of it. It doesn't offend me, nor is it hilarious.