I think it cuts so deep into people's psychology, and frequently religion. The very notion that we are not the apex of anything. And that we no longer need to eat other animals to live and be healthy. Too much to bear for many, so it frequently results in low-quality conversation, laden with emotions.
The fact remains, however. Non-human animals are not that different from us. Pretty much all mental behavior is represented in the animal kingdom. They, in so many ways, are us. And we are them.
Why should it be otherwise, after all? It would be strange to have a quantum jump in mental behavior with humans, and only primitive behavior in all other animals.
It'll probably take some centuries for humans to see other animals as inherently worthy of respect.