A while before the prequels were made, it seems like I remember media outlets saying George Lucas intended to "direct the first film" and presumably hand over the duty to others for the second and third episodes. After the poorly received Episode I came out, it was made known that Lucas would also direct Episodes II and III. Considering how everyone would have had a field day criticizing Lucas and his present-day abilities if another director had taken over for II and III and made vastly superior films, I believe Lucas did not relinquish directing duties in order to save face. He doesn't even like directing! He obviously knew that the remaining episodes would contain (slightly) more compelling material and likely wanted to preserve his integrity and respect by directing them, as well.