My husband Tom Jr and our son Tommy were playing a game on the PS2 called Champions of Norrath. They beat it on the easy level, and now are trying a more difficult one. At the end of this game, though, it asked this question- the title of this entry. It got me thinking.
The Sith had been gone for a millennium. Even Ki-Adi-Mundi said as much. As the events in TPM unfold, it seems the Sith are once again active, even though Mace Windu questions this. How could the Jedi not see this?
The Jedi kept the peace in the galaxy without Sith interference for a millennium. Therefore the Jedi felt the Sith no longer existed. As events unfold, we see just how wrong the Jedi were in their thinking.
Starting with ROTS, we see that the Sith have power in the galaxy. Palpatine is revealed to be Darth Sidious, the Dark Lord the Jedi have been searching for since TPM. The Jedi never saw that coming! Sidious recruits Anakin Skywalker to be his new apprentice Darth Vader.
As events unfold further, we see the Sith with more and more power and the Jedi cannot stop it. Enter Order 66. The Sith rule the galaxy unchecked until ROTJ. Luke Skywalker is the only Jedi left in the galaxy. It is up to him to defeat the Sith.
Luke tries to reason with Darth Vader (once Yoda confirms that they are father and son), but at first seems to fail. But Vader is confused at this point as to whom he should be listening to. In the end, Vader responds to his son's pleas for help and thows Palpatine over the side into the chasm. The Chosen One FINALLY brings balance to the Force.
But I say again- what is good without evil? They need each other to balance out the scales-one side to bring order; the other chaos. So long as there are Jedi in the galaxy, so the Sith will also be.

Thoughts or comments?