Because criminal justice reform would benefit minorities. And Republicans don't care about minorities.
If only it were that innocent. They of course care about minorities - negatively. They understand damn good and well that a lack of reform is "good" from their perspective in that it continues to keep down the people they dislike. Get a crowd of 100 of Naso's "working-class whites" in a room and ask everybody to stand up who would prefer to be black instead of white. None of them will, and it's because they know in their hearts of hearts (even if it's subconscious) that there's no "reverse racism" equivalent and/or auxiliary minority benefits that outweigh the benefits of being white. Add to that the simplistic thinking that equality or the lack thereof is always a zero-sum game and it's very, very clear.