I think it was believed that time that true sith were extinct.
They didnt know about the Emperor (and his sith forces) 300 years before SWTOR.
Quote
With the Sith Empire's defeat in the Great Hyperspace War, the Republic decided how to deal with their fallen enemy: the Supreme Chancellor ordered the Republic military to exterminate all remnants of the Sith. But one Sith Lord and a group of survivors escaped the Republic's slaughter of their kind and made it to the Unknown Regions. There, he re-founded the Empire, himself assuming the title of Sith Emperor, and created a council of other Sith Lords. Using the power of the dark side, the Emperor kept himself alive for over a thousand years after the Great Hyperspace War, rebuilding the Empire's strength and promising his people revenge on the Republic and Jedi.
This post was edited by A122 on Apr 4 2012 04:50pm