I think one of the reasons is because animals for the most part can't fight for themselves. People can.
I also believe we were all put here for different purposes. Some people are here to fight for human rights. Some people were put here to fight for the animals. They have no voice. If we all were here for the same purpose, the world would be even more out of balance than it is.
I don't think there is an easy answer to your question. There will always be injustice. I don't know what I, one person in the U.S. can do about the genocide. But I know I can help the cat I found hungry in the street. Blessings to you for caring so much.