Also interesting to note that Alan Dean Foster wrote Splinter of the Mind's Eye, which was written as a sort of backup plan for The Empire Strikes Back. If Star Wars flopped at the box office, Lucas would adapt Splinter into a low-budget sequel to Star Wars.
Lucas' decision not to use Alan Dean Foster for followons probably says more about Lucas than Foster. Once he realized he had a money printing machine on his hands, the last thing he needed was a name author with ego, and ideas about character development and story line.
That said, he got him back in 2002 and 2015. I guess by then Lucas could afford to be more affable about name writers and his universe.
Maybe it all came down to contract and authors rights? People say Alec Guinness negotiated a very good deal, they don't tend to say anyone else did.
