Fantasy settings rely on magic, wonders, and monsters to create extraordinary circumstances. It is the things that don't belong in the everyday world that don't just make it Fantastical, but give the PCs something to focus on. In Fantasy, either problems are caused by the magic and monsters, or can only be solved with them.
And in fantasy, you can draw upon pre-existing myths and folklore, as well as a rich tradition of pulp tropes to come up with the contents of your setting.
Modern role-playing settings rely on factions, contacts, and conspiracies to create something extraordinary for the setting. A well written conspiracy, a couple of secret societies, or a criminal organization, combined with some well done NPCs are what makes a modern setting seem like something special and worth pursuing. And again, it is these groups and NPCs that the PCs must focus on as the problem and the solution.
In a Science Fiction setting we have neither the familiarity of a modern or pseudohistorical medieval setting to work with from the beginning. And, while a good science fiction story might include conspiracies and criminal organizations, they require a lot of context to be made useful or understandable.
In science fiction, advanced technology, strange planets, and alien races create the extraordinary circumstances that drives the player characters: aliens, strange worlds, or tech that either create problems, or are the only means of solving them.
Because Science Fiction it doesn't have the context of the modern world or a historical period or mythological backdrop, that technology has to be custom designed by the game master.
I am going to cover the basics of the technology and how you can use themes to develop the technology in your setting below: