The greatest weakness of the Sith is that they are always plotting against each other. Not to mention by this point Palpatine had become overconfident. If Palpatine and Vader worked together they could easily have killed Luke, but both of them wanted Luke as their apprentice so they could overthrow the other.
Palpatine abandoned the banite system. He believed he was the culmination of the rule of two and used his apprentices for his own bidding instead of gaining power for the legacy of the sith itself.
Among other things, the master Sith invited one of his enemies into his throne room, with the hopes of turning him to the dark side of the Force.
Yet they were defeated by a bunch of grass smoking, guitar playing, Ewok cuddling hippies. Among other things. Hello? Did you pay attention to the movie? Why can't critics just be straightforward with their criticism instead of concealing silly insults inside the punchline of what would otherwise be a legitimate question?