Why is Palpatine never referred to as Sidious in the original trilogy?

In the original trilogy, Palpatine is referred to as Sidious once, by Darth Vader in Return of the Jedi. After Luke has revealed that Vader has a son, Vader says "You don't know the power of the dark side. I must obey my master." To which the Emperor replies "Obey your master, yes. How wise you are, Vader. Now go, my apprentice. Leave this task to me."