I think that the Clone wars and then after tge fall of the Republic years of oppression by the Emperor will have caused the Empire to fall into a poor state, the Republic never had a real need to fund a massive army and Star Fleet to control the universe. The Emperor puts a lot of the universes resourses into The fleet and arms to control his 'loyal' subjects. That with the ongoing battle with the rebels whos limited resources meant they had to cope with lesser technologies than the Empire will have made it appear thus.
hard times opens for discover better and cheaper ways of producing things. I would say the technology in the OT is higher or at least it is meant for it to be higher.
You must take into consideration on when the films were made, today it is easier to make things look advanced than it was in the time of when ROTJ was made