You are mistaken, it does fit. The sith empire are the ones who fled korriban after the great hyperspace war, mostly human and red skin sith, who intermarried. The redskins you see know are human with some or little "sith pureblood". So they are the main power and have been isolated for a long time. The aliens they have encountered have been eitehr slaves or conquests. Most other aliens are from the republic, so yes the speciesm is understandable