Marx wrote about a theory of history in which all of history progresses through phases in which each phase is defined by struggle between classes. His view was that communism was the inevitable end of this model of history in that communism would finally resolve the class struggle. But for somebody who wishes to promote communism, the problem is if there is no class struggle, then history does not advance to their desired end state.

The original formulation of Marxism was economic, based on the struggle between the proletariat and the bourgeoisie. This conflict worked to create communist revolutions in nations with more feudalistic systems like Tzarist Russia. However Communists have struggled to create revolution in more Democratic societies with greater social mobility, or at least the perceived possibility of.

So failing to get the economic class consciousness to trigger a revolution, the modern Marxist has moved on to pushing race/culture based class consciousness as the trigger for their revolution. This is why todays Marxist is not obsessed with rich and poor but white vs black vs hispanic.