Didn't Lucas say after ROTJ the Sith are destroyed and NEVER return?
Continuity.
In 1983 when RotJ was completed, Lucas did not have the term "Sith". The Emperor and Darth Vader were just Dark Side users who opposed the Jedi.
The prophecy was non-existent.
In 1990-something, Dark Empire came out and resurrected the Emperor - and George Lucas praised the comic, and said that it was the closest thing to a "sequel" of Star Wars (he and I disagree).
In 1998, he began to map together the back story. The prophecy was born. The Sith was born.
Unfortunately, by that time, too many comics/works had used the Sith and Dark Side Force users, which made Anakin's struggle somewhat in vain - though he did technically destroy the most effective Sith and ended their greatest reign.
Doesn't that blow the whole EU "Sidious returns uber-powerful" stuff out the window?
Ogami, this, unfortunately, makes no sense. Sidious did return, uber-powerful, and far stronger than he was in the movies. I dislike DE, and all other 'superpowerful characters', but this just isn't the case.