The Shadow of Victory
Following the Jedi Civil War, where Darth Revan returned from the Unknown Regions with a Sith armada, the unified Sith forces fell apart.
With the defeat of Darth Malak the Sith had no leadership, with individual Sith Lords declaring independence. The Sith holdings fractured into hotly contested kingdoms.
While not unified, each individual kingdom is strong enough to defend itself, preventing the Republic from stamping the Sith out.