It makes sense 'cause they're already prone to denying the holocaust and centristly trying to go "well I'm sure the truth's somewhere between 0 and # million'. And since it's basically the same mode of production to do it and they can't imagine society letting it happen for some reason, they'll just deny it.

Edit: Also does first-world soy-conservative thing of taking an awful thing that's happening to someone else and flipping it to actually be about themselves instead, i.e. meat-eaters are being tricked into eating whatever rather than millions of creatures are being converted into food stuffs.