Why do religious leaders complain about humane treatment of animals because it clashes with their barbaric animal slaughter rituals?
[bbc.com]
Because they believe in the thin end of the wedge. If they allow even the smallest interpretation of their holy books, even if it is requires the most extreme interpretation in the other direction, to make it seem conflicting, then it raises for them the possibility that people may try to ask more questions, or even ask who gets to do the interpreting, and why. "And God forbid that."
Simply, because their ignorant god said so? This illiterate and uninformed god set humans apart from the animals, as opposed to teaching what we now know today--namely, that homo sapiens evolved from lower forms and we share a common ancestor. Once humanity fully accepts the knowledge that evolution imparts, including the implications of the range of suffering that all animals experience, a thoughtful rethinking of these so-called divine commands may take place.