I always thought that intellectual laziness was a large factor. It's much easier, cognitively, to judge someone by his or her race than by achievements and character.

http://aynrandlexicon.com/lexicon/racism.html

> Like every form of determinism, racism invalidates the specific attribute which distinguishes man from all other living species: his rational faculty. Racism negates two aspects of man’s life: reason and choice, or mind and morality, replacing them with chemical predestination.