You can largely blame Disney for that one. Lucas has been extremely explicit and direct that the dark side and the sith are a corrupting influence on the natural balance of the force and the universe the Jedi serve.

Lucas later retconned that, voluntarily, in a terrible Clone Wars episode involving an evil moon, a good moon, and a pompous magic family with a daddy figure lecturing his kids about how they each need to have just enough evil in their diet for a balanced breakfast.