What if Anakin's spirit cannot progress beyond a certain point in the spirit world until Leia forgives him? Anakin's spirit could spend the whole trilogy guiding the heroes, in an attempt to earn Leia's forgiveness, as her negative feelings towards her father, manifest in the Force as Dark Side energy, is being used against the New Republic. In the end, perhaps it's the forgiveness itself that weakens the main villain and finally brings lasting balance to the Force. It makes sense when you think of the Skywalker twins as the two last hopes of the galaxy. Luke's challenge was to reach the buried love in his father, and Leia's is to find forgiveness.