Probably showing my inner geek, but I remember reading some of the Star Wars Extended Universe material where they explained the reason for that. Humans (or the humans of the Star Wars Galaxy) where the first species to discover hyperspace travel something like 100,000 years before the current time-line. Humans then colonized much of the galaxy, while they would engage in limited trade and contact with alien life forms they encountered, they refused to provide the secret of hyperspace travel and other advanced technology. So aliens were stuck in their home systems and greatly limited technologically. It was only in the last 10,000 years or so that hyperspace travel began to spread among other species. That is why, in the Star Wars Universe the movies are set in, Humans are still, by far, the largest and most dominant species and have colonized many worlds across the galaxy, while alien species, for the most part, are still limited to their home systems. That also explains why you see vastly different technological levels, even among alien and human residents of the same planet.
It sounds like the Empire spends a LOT of money to maintain control over galaxies full of interstellar Ghanaians and Navahos.
10,000 years is a long, long time. 10,000 years ago, real humans first started to farm. Imagine where we would be today if back then they were given tech from 200 years from now. I doubt they'd still be wearing skins and writing with sticks in mud.
But I recall reading a short story once that was very interesting. A group of teddy bear like creatures traveled to earth in a rickety space craft which mostly resembled the interior of an old sailing vessel from the 18th century, no plumbing (they used pots) or anything else techologically advanced. They landed in central park and walked out carrying flint lock pistols to conquer earth and saw sky scrapers and tanks waiting for them and as they were being walked off in chains the captain had an epiphany. Star drive was so simple, so basic that everyone got it very early in their existence and it became the basis of their whole civilization. They never advanced once they got it. But somehow earth missed this obvious thing and technologically humans were 1,000 times more advanced than anyone else. The only thing stopping them from easily conquering the whole galaxy was their lack of star drive. And now they had one....