Depends on what you mean by minorities but prepared minorities do use test prep. Poor black kids at disadvantaged high schools in my experience never did.

I really meant the economically depressed people I knew when I was a kid in my urban high school in the south. My parents went to college and knew about preparing for those tests (like taking the psat and prep books, that's as far as I went). But plenty of my fellow students at my high school never really escaped the urban environment and didn't get a chance to go to college and beyond. If their parents were more prepared for this world, or they were lucky and able to navigate it on their own they might have had very different lives. I was not particularly savvy about the college world but my parents knew what to do. There are no doubt plenty of asian immigrants who also didn't have those parents who helped them get started in colleges, just like non-immigrant americans had parents who didn't help them get out to college.