The same reason that people insult others over the fact they adopt a different religion or that they they come from a different country or they have a different political belief or have a physical abnormality or a learning disability or use rhetoric that others disagree with or are just someone...
As indeed with all the other human rights abuses around the world and the racism and racialism found in every corner and in every nation on earth and not just an individual issue with colour prejudice, which some seem to consider the only cause of importance.