Because of the huge, huge carbon footprint of animal agriculture. I was shocked to find out that animal agriculture directly or indirectly accounts for 14.5% of all greenhouse gas emissions, compared to all transportation – every ship, car, truck, plane on the planet only accounts for 13%. Less than animal agriculture. So most people think that buying a Prius is the answer, and it’s certainly not wrong, but it’s not the biggest agent of climate change.
From an " Ask Me Anything https://www.reddit.com/r/IAmA/comments/22uz4m/i_am_james_cameron_ama/" session on Reddit; as quoted in "Director James Cameron on Vegan Diet: Like I've Set the Clock Back 15 Years", in Ecorazzi (12 April 2014) http://www.ecorazzi.com/2014/04/12/director-james-cameron-on-vegan-diet-like-ive-set-the-clock-back-15-years/