Isn't it self explanatory. Star Wars was becoming a joke in Lucas' hands. It was time for new blood, new way of thinking, new direction.

I can assure you of this. I grew up on SW and I have an understanding of the material and the man behind the curtain that far reaches most casual viewers. I understand his way of thinking.
If left to his own devices, he would have continuously damaged the good work he did in the late 70's when he was under the protective eye of the studio and not later in his life when it was George Lucas unbound with Jar Jar Binks and midichlorians (which by the way demystified the Force in one fell swoop)