Because it doesn't jive with the vegan agenda. Look at the most upvoted comments here. It's all stealth vegan propaganda masquerading as concern about the environment.

Never mind that cars, ships, agriculture ( I bet most of these vegans haven't ever heard of petrochemicals or fertilizers ), smartphones, electricity, etc are all individually greater contributors to environmental damage.

Also, the growing numbers of bison, deer and other ruminants around the world contributes to increase in methane levels. People forget that all ruminants ( bison, cows, etc ) all produce methane though they'll use the faux stat of them being net negative contributors.

And sad as it is, veganism will actually increase fossil fuel usage as the vegetation that people eat need petrochemical fertilizers to grow and a vegan diet cannot be locally sourced like meat can, so they need to ship vegan food from all over the world. But vegans don't like facts too much. They'll throw in the nonsense about livestock agriculture being transfered for humans. Good luck surviving on corn, soy and wheat. Humans aren't ruminants. We are omnivores by nature.