Hang on, Hang on.
The Sith Lords went way back before Darth Siddious(The Emperor) was even a gleam in his Mommy's eye. They didn't lose, their faith continued over millenia. The struggle for the supremacy of the Darkside was not lost, it has just been delayed.
There are only ever 2 Sith Lords - how many whimpy Jedi were needed to combat just those 2? Sounds like the usual odds for an arnor builder.
The Dark side was supposedly the bad side......but how many people loved Darth Vader. Statistically he was far more popular than Obi Wan and Luke combined. (Leia was fairly popular with the young boys too, I hear)
The Darkside is the faith of the dedicated and gifted few, not the misbeggotten field of dreams granted the multitudes. Sheer weight of numbers does not always win. Anyone heard of Rouke's Drift?
The Darkside shall yet reign supreme, and the followers of the Light side shall forever be banished to the Nether Region of cleaning up the seams of our road wheels and the purgatory of building our Aber 3 piece German tool clamps.