Light leaning Sith are mercilessly hunted in the Empire. What few exist either conceal their true nature from others or die. Not that a light Sith couldn't do good deeds, but there's a limit to how far they can go with being noticed.
The main difference here is that Keleth'Ur heavily implies that he didn't conceal his true nature. Rather, he fully embraced it and tried to change the Sith dogma, which supposedly brought down the Emperor's wrath.