It's a year after the release of the movie Star Wars and the Role playing public has a taste for role playing adventures in space and distant solar systems with aliens, starships and lasers!  And, sadly, there wasn't much there.  As I am told, and I believe reliably, a guy named Marc Miller rolled a filing cabinet into the GDW pitch meeting on a dolly.  When he was asked about it, he explained his interest in presenting such a game, and the file cabinet was only one of a number of them filled with his pre-design research.
From there, GDW put their shoulders to the grind stone and worked on winnowing down all the data Mr. Miller had to something they could present as a product.  And they called it "Traveller".  It came in a small black box with an image of a starship, the Free Trader Beowulf, calling...begging for help while under attack!
Soon, there were more source books, a periodical called the "Journal of the Traveller's Aid Society" and the birth of the "Official Traveller Universe" (OTU).  From there, the game, and releases based on it, grew and a number of these actually won the Hugo science fiction writing awards!  But what really was Traveller?  In the end, it was "the same characters" in any setting well before there was something called "GURPS".
A simple system let a game master create any solar system, or any number of solar systems.  And, where one system could be populated by a culture having only medieval technology, the next system over a few parsecs could be comfortably interstellar.  And those with starships could visit any of these, or even all of them in time.  So long as they could get the food, fuel and other supplies needed. And just how much would a medieval culture pay in precious minerals, gems, artwork and other valued items to be able to ride their horses against their foes carrying highly reliable machineguns?
Because in the end, people are people.  Yes, they can travel the stars, but they still love, dream, aspire and struggle for their interests.  Where there is love there is also lust, be it for a partner, wealth, power, ownership of a specific object or even intellectual property.  Or just for the love/lust of adventure.  These sophonts, being Human or alien, all have the same drives we have every day.  Their only difference from every person you know is that their love or lust drove them to step outside a normal life and travel the stars.  What would you travel into the unknown on a starship for? That's Traveller
For more on Traveller, click on this link to visit Far Futures Enterprises for the current Traveller             version(T5).  Or, Visit Mongoose Publishing for their licensed version.