With dozens of short stories and novels written by scifi icon Larry Niven and an awarding winning group of visiting authors, I noticed that Star Trek TAS episode Slaver Weapon is the only TV show or movie to visit Larry Niven's Known Space. Why haven't there been more page to screen adaptations?
I am slightly hesitant about answering a question like this, as most answers are likely to be subjective unless we can find an interview from a movie industry insider who says that Larry Niven stories are hated - which is very unlikely even if it were true.
I think that the problem is that Niven's stories are just not that easily adapted for the screen. I don't mean that I think they would not be cinematic - I would love to see Hollywood's vision of a Ringworld, or of the Smoke Ring and personally I love Niven's novels and short stories - they are great hard sci-fi and full of ideas. However I think it would be very hard to write a script against any of them without significantly rewriting them. Niven does use a lot of internal monolog and dialog to explain stuff, particularly the internal monolog is quite hard to turn into a movie. Then there is the issue that so many of Niven's stories are intertwined and are best appreciated if you've read the whole canon. Movies have to stand alone, and bringing in all that backstory would take time.
I think that Ringworld has been optioned before to be turned into a movie, but has never happened. If you look at the story it is a contrived series of events just to allow us to explore the fantastic idea of a livable ring around a star. If you follow the story as action however it is fairly dull - they assemble a team, they go to the Puppeteer home-world, they go to the Ringworld, they crash, they fly across a tiny fraction of it, have a few adventures, find some stuff and escape. Ringworld Engineers has a bit more 'umph' to the story, but it is very complex for a single movie and would have to start to tie in Protector too. They are great novels, but I think would probably make either very long or very confusing movies.
The best science fiction movies tend to have either no source material, or a relatively stand alone and a short-story or brief novel as a source. Of Niven's works, probably the non-Known-Space stories would be better candidates as they are designed to be stand alone works.
Philip K. Dick is probably the most visited sci-fi author when it comes to being source material for movies. There are probably many reasons for this ... but the fact that he wrote a lot of short, stand alone stories, with a relatively simple 'high concept' idea in them that can be adapted for a movie without religiously following the plot is probably a large part of that. The Soft Weapon story by Niven which was adapted for ST:TAS as The Slaver Weapon episode is a very close parallel to that.
Larry Niven wrote The Slaver Weapon. At the time, Roddenberry thought The Animated Series would essentially be Star Trek's last hurrah. He has said that he did it with Filmation because he thought it was the only chance he'd ever have to do another series. And they allowed some well known SF authors like Niven to come and work with them (as they did in the original show).
Before that, there were no universe crossovers for the usual reasons. The first is royalties. There's also issues like compatible universes. If you are writing a live action show and start crossing over into another complex universe created by another writer, then that writer can pull the rug right out from under you with one book or short story. You, as a producer of an expensive TV show, have no control over the property someone else created.
Until Niven came in and said, "Have I got a story for you!" there was no reason to do a crossover. But when Niven was there, they had the chance to work with an SF writer who was already a legend, it was just a Saturday morning cartoon, so many people wouldn't give it much credit anyway, and since Niven wrote the story, there may have been a deal that was cheap and easy in terms of him allowing them to use elements of works he had already created.
After that, though, Trek was back on top. The movies were a significant budget and to use anything else from the Known Space series, they would have had to negotiate royalties, as well as making sure that the Trek universe dovetailed with Known Space. (While I've only read a bit in that universe, though, I think they use a very different FTL and the universe works quite differently than in Trek.)
And on a more practical level, writers love to make up and tell stories -- their own stories. If you're working with a show like Trek and all the characters in it, as a writer, you're going to have a lot of your own ideas of what stories you can tell, so there's not only no need to borrow from someone else's work, but if you do, then there's less time to tell your stories.
Although not from his "Known Space" future history, Niven's "Inconstant Moon" was made into an episode of The Outer Limits in 1996: http://www.imdb.com/title/tt0667911/