When have the Sith had a LASTING victory over the Jedi in all of Galactic History? A few years between The Jedi Civil War and the Triumvirate's Interregnum, a few years where they had the upper hand between the Great Galactic War and the breaking of the Treaty of Corrosant (we all know the Republic wins in the end of this little war we play out in TOR), two or so decades during the Dark Times, and a few years during One Sith Empire. The rest of the time they are either actively fighting the Jedi, licking their wounds after loosing to the Jedi, or "planning" for their next fight with the Jedi.
Jedi on the other hand go entire generations between serious Sith threats in a good run.
That, my friend, is SIMPLY because Lucas, & the creators of those games ultimately kowtowed to the conventional wisdom/societal pressure that 'good' has to win out. BS.
If SW were reality, a militarist, strongest survive mentality run society & nation like the Sith Empire would likely, ultimately, eat the Republic for lunch.