Because people believe that social class is your own fault. That people can just pull themselves up by their bootstraps if they weren't lazy. Not that I agree with this.
But the resistance to dealing with race and class together is real, in part because class is seen as individual problem. Race is seen as more cultural, systemic, or the province a minority running around with sheets on their heads.
I've personally only seen strong efforts to dissect them both, in earnest, at the same time, in the margins of society.