(or "Parts Do Not An Engine Make")
When the Star Wars saga resumed, in 1991, it was with Timothy Zahn's sleek thriller 'Heir to the Empire' (and with the rejoicing of a generation of grown-up Star Wars geeks.) The series picked up five years after the end of the classic trilogy, featuring a sinister, calculating general in the mold of Peter Cushing's Grand Moff Tarkin, with a plan to restore the glory of the Empire and defeat the hated Rebellion.
The "Thrawn trilogy" proved popular enough to unleash a torrent of spin-offs, from comic book series like 'Dark Empire' (featuring a reborn Emperor), to 'Knights of the Old Republic' (featuring a Sith Lord, Exar Kun, and Ulic Qel-Droma, the Jedi he seduced to the Dark Side) to novels like 'The Courtship of Princess Leia' (featuring an Emperor-like Dark Jedi named Gethzerion), to the 'Jedi Academy' series of novels (featuring a Dark Jedi whom Luke must help redeem), to 'Darksaber' (featuring an evil Hutt who is Jabba the Hutt's successor) to the "Rogue Squadron" series of both novels and comics (featuring Ysenne Isard, another Tarkin-esque figure), to the massive 'Shadows of the Empire' project, which crossed over into just about every medium other than film (and centered around a Jabba-esque crimelord...)
Do you start to see the issue, here? It's not fair to suggest that the ancillary Star Wars stories are derivative, but the villains for a lot of the spin-offs do tend to fit neatly into the mold of either being like Tarkin, Vader, the Emperor, or Jabba the Hutt...coincidentally, the four main villainous characters in the classic trilogy. The series seems to be trying endlessly to replace the villains Lucas finished off in his final movie (or, in the case of Tarkin, his first.) Some uncharitable types might say that it's because the authors who've written for the Star Wars spin-offs are unimaginative, but I think it's actually down to the nature of the universe they're writing for.
Because Lucas didn't intend for Star Wars to be a storytelling engine. Sure, he put in tons of detail--the series is known for the thousands of tiny little elements, in the dialogue, the costuming, the special effects, and the entire aesthetic of the series that all combine to form a galaxy that seems limitless and filled with potential. But Lucas didn't do that to create an engine to tell lots and lots of stories--he did it to make the one story he wanted to tell seem immersive and real. The Star Wars movies aren't open-ended; they're the tale of Darth Vader, and his fall and redemption. Details like "the Kessel run" and Krayt Dragons and bacta tanks and tibanna gas and the temples on Yavin are there for verisimilitude. Even the title, 'Episode IV', was put there originally just to give you the sense that you've entered into a story that has a history. (Lucas, of course, claims that he had Episodes I-III planned out all along. But when you have a draft of 'Empire' that has Lando being a descendant of the evil clones that the Jedi fought during the Clone Wars, it does make that claim a bit suspect.)
The key sign of the lack of a deliberate, designed storytelling engine in Star Wars is the lack of villains. Lucas designed his story to have a beginning, middle, and an end, and he neatly made sure that all the major villains of the piece got their comeuppance by that ending. And there's nothing wrong with that, of course. Nobody has to plan for a series to come out of the story they want to tell, and nobody has to think in terms of a potentially endless series of sequels and spin-offs. Lucas told the story he wanted to tell, and in a very real sense, it was the audience who demanded more stories. One year's fan became another year's storyteller, filling in every detail of the universe Lucas created with meaning. But it's much easier to fill in the background of the people, places and things that make up the Star Wars universe than it is to make entirely new stories set there. It's not too surprising, really, that they turned to the models Lucas had created as inspiration for their stories.