I don't know how to put it in words exactly, but "future is doomed" is somehow "everyone knows" one way or another (at least I am going to die before doom happens and a single individual cannot be held responsible for the doom anyways).
And then we also have the factories polluting the environment where we can debate on how useful the end product is, but when it comes down to a person eating meat the discussion just never would convince meat eaters in my opinion. Meat eaters could say "who's ultimately benefiting from the factories polluting environment" and the answer would more or less be some corporation. Why should they have all the fun ?