I'm a lawyer, so I'm going to approach this from a lawyer's point of view. The modern legal system ruins adventures. And that's why fantasy is much easier to make an adventure for than science fiction, because fantasy almost always assumes a world with a non-existent legal system, while science-fiction almost always assumes a world that resembles our modern world, except with high tech elements thrown in, and that includes a modern legal system. And in the modern world, the one we live in, the legal system ruins adventures, and so it would be in your typical science fiction setting.
What I mean by that is that if your neighbor's cattle are drinking from your water hole, you don't round up the boys and have a shoot out to resolve things. You hire a lawyer, and you sue your neighbor. If your daughter is kidnapped, you don't load up on ammo and strap on your bullet-proof vest and go catch the killer, you call the police and have them look for the killer. Now, all this can be hand waved in a movie or TV show, because the cops conveniently never investigate no matter how much property damage the characters wreak, or how many people they kill in their quest for vengeance, but in the context of a role playing game, this lack of realism will quickly stack up to an unbelievable adventure. The fact is, in a modern setting, or any science fiction setting that resembles the real world, a SWAT team will show up in minutes and quickly ruin the combat the PCs are having with the villains.
There are ways to solve this problem, you place the adventure in a place that is on the outskirts of civilization, and have a Wild West type campaign where law enforcement may be only one sheriff 100 miles away. Or you put your campaign in a setting where the government is unjust, and law enforcement is corrupt, and so the characters have no choice but to take the law into their own hands. Or you put your campaign in a setting of constant civil war, where law and order have broken down, so the characters have no one but themselves to turn to when there's a problem. Or the main characters can be criminals, and fighting the law is the main element of the adventure.
Star Wars solves the problem with three approaches. In Edge of the Empire, characters are people who are on the boundaries of the law, in a Wild West setting. In Age of Rebellion, characters are caught up in a civil war, and the war drives the action. In Force and Destiny, the characters are hunted fugitives, unsafe even on the core worlds from the possibility of arrest by the authorities.
The reason Traveler doesn't work so well, is because it assumes a stable government, with well enforced laws, and the government is mostly benign. Traveler adventures involve loading up on passengers, mail and cargo, and moving them to another planet, while paying the mortgage on the ship, which involves a lot of bookkeeping.
I created a science fiction campaign which started with a terrorist incident that threw the entire Imperial government into a state of chaos, and led to a civil war. The characters spent the entire time fleeing from one planet to the next, staying just a few steps ahead of the evil Home Party, which had overthrown the Emperor and was now clamping down on all space travelers and throwing them into detention camps. It was a frantic chase, and it worked, but it was a lot harder to create this adventure than a simple fantasy dungeon, with traps and monsters.
