It really does seem that the slaves were a subplot that was supposed to be dealt with later on in the series and simply didn't get around to being included, or a 'deliberate creative decision' was made to drop it. But as said before, the late years of the Old Republic wasn't all that good. If the Republic hadn't grown corrupt and ineffective, Palpatine wouldn't of been able to take over, and engineering the clone wars would of been nearly impossible. But the slaves should of been dealt with, and I imagine that there will be a book or comic series that will have a pre-darkside Anakin returning to Tattoine and freeing the slaves, if there isn't already one out. The EU is pretty good about taking the tiniest little point and turning it into a major event, so something like the slaves won't escape their attention.