After the fall of your empire, the Sith were forced to go into hiding. Although their numbers were greatly reduced, they bid their time until they could overthrow the Jedi, who believed them to be fully extinct.

[She could go on and on about the failings of the Jedi. Her existence was not proof of the Sith's strength, but a result of the Jedi's weakness.]

I believe the Sith of my era fear the repercussions that may come if they were to rebuild their Order. As you said... I'm not strong enough to be a Sith Lord. That is by their design, not by my virtues.