"For over a thousand generations, the Jedi knights were the guardians of peace and justice in the old Republic" Maybe I am reading too much into this phrase but I always assumed there were numerous before the Empire. In Revenge of the Sith...Palpatine said the Jedi attacked him. He used this to fuel the perception that they were terrorists and charlatans. Everything related to the Jedi was destroyed and public opinion turned against them. This is why 20 years later few people know about the force. An imperial officer can mock Vader to his face.