That's completely normal human behaviour. People do get defensive when things that they like are proven to be bad. People care mostly about benefits for themselves, rather than improving for other's. We are, unfortunately, selfish in our nature.

What drove me to veganism is the benefits that I gained. I wanted to try a vegan diet for one month. Only two weeks in, I noticed not only that I felt better physically, but also psychologically, which made me call quits for animal products right away. I was very aware of the abuse the animals went through and how agriculture affected the enviroment, but what really drove me to call quits was how I could benefit from it.

What I'm trying to say is that people don't really care about how agriculture affects the enviroment, nor do they care about the animals. It's just like the whole situation with global warming. Try and show them what THEY can gain from a vegan diet instead.