It is indeed about morality. Morality is about what is “good” and “bad”, so it’s perfectly in line with OP’s question “why is the consumption of meat considered bad”.

Religions have arbitrary morality so it doesn’t seem very interesting to discuss why these religions allow or forbid to eat their specific set of animals, unless you’re studying these religions.

Moral philosophy on the contrary tries to study morality with real arguments. In almost all cases they agree it’s bad to harm others while it’s not necessary. Even with our intuitive morality most people would agree with that. And in most cases eating animals products contributes to harming them and is not necessary. It was not necessarily the case in the past, but today it is. So eating animal products nowadays is immoral.

The environmental problems only adds additional harms on top of that by causing harms to even more animals, including humans.