The Eu tries to fill in those plot holes and discontinuities. But the average filmgoer is not going to watch all the films, read all the comics and play the videogames to decipher the plot.
They also would not have read those prequels to the phantom menace novels that described the sith, or the politics of the trade federation, or palpaptine's setting up Chancellor Valorum so he could become chancellor and then emperor.
The sith history is still being written for instance in the unreleased as of yet old republic mmo, or the recently released third novel in the darth bane trilogy. These days for star wars to make sense you have to almost digest 100% of the EU and LFL products. Imagine not telling the clone wars in the movies so they could milk it for all its worth.
The prequels are a boring 6 hour skeleton plot, with poorly developed cardboard and cartoon characters, and scripts that should have had a least a few more drafts and less yes men. It also has too much greenscreen and cgi and makes the movie feel sterile and inhuman and not a real craftman's work of art which the oot was.
To expect one to read tie in books to make a movie make sense is just about the most lazy form of filmaking i have ever heard of. If you can even call those video sequels films that were not shot on film.