Because animals are innocent sin and of all the other assorted bits of vileness that a sentient mind is capable of creating and the vast majority of humans simply aren't innocent. Children are untainted for a while but they all eventually mature and end up becoming part of the problem. That's why animals deserve pity but people don't.
Give it time. In a hundred and fifty years or so, thanks to the attitudes created by 'God gave dominion over the world and everything in it to man' bullshit that is in every religion ever created, practically all the animals are going to be extinct at our hands.
Posted it before, posting it again;
$1:
"In relation to them, all people are Nazis; for the animals it is a holocaust, an eternal Treblinka."
“The worst sin towards our fellow creatures is not to hate them, but to be indifferent to them; that's the essence of inhumanity.”
- Issac Bashevis Singer
In reality, if the planet and the non-human creatures on it have a best interest, as such as they're capable of conceiving such a thing, it would be if the entire human species disappeared altogether. There isn't a single non-human species that wouldn't rebound and then thrive if humans were no longer holding the sword over them.