I'm pondering the whys of how this has come about, and I suspect it's yet another example of America's assumption that its own cultural narratives are globally applicable.
I'm not hugely up on this bit of intellectual history either, but some marxist writers I have read who study race in America have said that this has gone back and forth somewhat, between a view of race as an effect or expression or mechanism of class, and then this sort of racial essentialism. So racial essentialism was challenged after WWII, but had something of a resurgence in the McCarthy era before dying down again in the early civil rights era, but then kicked up again at the end of the 70s and into the neoliberal era where it's become the dominant view.
They see that as being a way to avoid challenging class in a more fundamental way, and also as supporting an activist/managerial class that has grown up in many marginalised communities. Because if you understand the material function of race as creating a class of workers for shit jobs, or who will be the unemployed, and that function of class in turn continues to create race as a category, you could begin to act on that effectively by challenging class, or the underclasses could band together to create change.
If race is a wholly separate thing with an existence of its own, OTOH, that protects the capitalist status quo.