This first depends on the amount of content.
Take Firefly. Lots of people love this ISP. But it's only a short TV show, a movie, a couple novels and comics....and a long lost out of print rpg book. So even if you have all of that...well, it's not too much content to use for a game. Alien is much the same a couple movies, novels and comics and an RPG book every decade or so....and that is it.
You're missing the impact of the comics... content equal to 2-3 more seasons of the show, and considered canon by the Brothers Weedon.. Not a lot really needs be added to be playable.
Trek had a huge amount of non-canon material - some licensed, some fannon. If anything, FASA's issue was that they didn't have access to their primary preferred source, and so made parallels...
Canon was 79 released episodes of TOS, 22 episodes of TAS, and 2 movies. Total canon content? About the same as Firefly, provided one counts the comics.
The thing is, Trek had a lot of non-canon material, which the designers' note lament inaccessibility of under their license. but some made it in by back doors... John M Ford writing novels and supplements, as exemplar.
And the tech manual's Saladin and Hermes having the FASA-Trek Larsen and Nelson, differing in the nacelle struts, cheating their way into the beta canon.
But that secondary material was off limits, and the bulk of it post-dates FASA Trek.
LUG lamented likewise, but with more series and movies.
Star Wars, when WEG took it up, had 3 movies, a TV special, 2 seasons of bad kid's cartoons, 8 years of daily comics, under a dozen novels. What we so much take for granted as the SW Extended Universe was largely WEG's work - and Zahn noted that he was referred to WEG's RPG and Sourcebooks as they key canon reference. Essentially, WEG created the renewed demand Demand which lead to the (IMO, mediocre) prequels, and lots of novels.
If Firefly as a game weren't killed off by MWP closing games, it could have done for Firefly what WEG did for Star Wars.