I think that’s because the Sith race eventually gets distinguished from the Sith “order,” which further gets boiled down to the Rule of Two stuff. The red skinned Sith race probably have more group solidarity than the later Sith which are just whoever claims the title, or the Rule of Two Sith where you’re literally supposed to kill your master. Still it’s a shame that modern fans only know the stereotypical kind. Supposedly in the Old Republic MMO game you can play as a “light side Sith” somehow and the Sith empire gets diluted and loses its edge by getting more political and less focused on being evil, but I think that’s lame.
It’s also interesting how Marka Ragnos’s ghost shows up to try to keep them together.