So I guess the big reason he directed it himself was pretty straightforward then:
He started off as a director and then became the man in charge of a company. When he finally had enough money to self-finance a prequel trilogy, he wanted to make sure every decision being made on the movie was a sound one, business-wise. So, why not go back to being the director instead of going through a middle-man producer?
Yeah, never thought of it that way.