[C Of K]

I once saw an interview with George Lucas. He basically stated that even during the height of the Empire, while Anakin was Darth Vader, he was still the chosen one.

The force was mainly out of balance because the jedi were serving evil. Even before Palpatine rose to power, the senate was corrupt, yet the jedi still enforced senatorial rulings. During Palpatine's rise to power things only got worse. The jedi were servants of a sith lord. Hence the statements that, "There are heroes on both sides. Evil is everywhere."

I don't think it had much to do with how many jedi or sith lords there were.