One major reason is that they buy into the white-racist ideology and like white privilege as they see it. Conventional electoral wisdom discounts race as a political factor, focusing instead on class, the gender gap, union membership, etc. All but union members and big-city residents are racial or ethnic groups. White union members and city dwellers vote to the left of whites who live more racially isolated lives, but they barely tilt Democratic. Similarly, women voted 54-43 for Gore, but white women actually favored Bush by one point.
Women of color create the gender gap.
The income gap in presidential politics is thoroughly racialized. As the sociologist William Form pointed out long ago, if only a bare majority of white working-class people voted consistently Democratic, we could have some kind of social democracy that would provide much more social justice than the conservative regimes we are used to.