In the forward to the Splinter of the Minds Eye book George is quoted in editions of the book as saying "That his story was too big to fit into one movie, and that he broke it up into three parts, The Original Trilogy, the further adventures, and the back story making a total of nine films"
I can well believe that there was more than 6 films planned back then, I just think he changed his mind, ran out of story or got fed up with SW at that point....