That was discussed to hell and beyond, but anything to movement the board and procrastinate even more.
So, why Vader never cloned any spare parts or at least change those prosthesis to something better, more advanced? A lot of EU material tell us about how shitty the suit was. So the 2nd most powerful man in the galaxy was forced to use a 2nd rate suit forever? Even with the explanation "Frank designed the suit to be shitty", Vader would at least changed his arms and legs and Palpatine would be none the wiser.
Some novel claim that he would move to a new suit, but his life support would have to be disconected and that was too risk… but there is also plenty of material showing him naked, without the armor, inside to chambers of his.
In a galaxy that can create entire clone armies and Death Stars, they can`t grow some spare limbs? Weird.