I had always thought of it this way: With the characters always talking how the Force is strong with so and so person, I had attributed the imbalance in the Force to the fact that Lord Sidious was so powerful as to be the embodiment of the Dark Side. When he turned Anakin into Darth Vader, the Dark Side became all the more potent with Anakin being a prodigy among Force-sensitives. Further, Mace Windu had acknowledged that the Jedi's ability to use the Force had diminished. In short, Jedi were getting weaker, and the Sith were getting stronger.
Combined, they were more than a match for the Jedi Order at the time the Clone Wars came to an end. After Order 66 reduced the Jedi numbers from thousands to fewer than 20, the contrast between the light and dark power became rather stark.
When Luke comes of age and is trained in the Jedi Arts, that in itself BEGINS to bring balance to the Force. Luke appeals to the shred of Anakin that remained in Vader by accepting that Vader was his father. Later, when Sidious is about finish off Luke, Anakin decides he will not allow his son to die and destroys the Emperor. In doing so, he returns to the light, vanquishes the personification of the Dark Side of the Force, and thus brings the Force back into balance.
