On another forum somewhere, I saw this question: “Was there ever any talk about why there weren't any alien Rebels (besides sort of Chewie)the early ANH scenes? Budget issues? I know we know there were different species in EU material though.”
My response: It was not yet established at that point that it was anything other than a Human Rebellion within a Human Empire. (It is idiomatic in Sci-Fi that different starfaring races would have their own empires elsewhere in the galaxy.)
That’s also the only way to make sense of calling other species ‘Aliens.’
RotJ is where they really made aliens seem mainstream — it’s where we got mon calamari, sullustans, hutts, twi’leks, gamorreans, ewoks, and the mention of bothans.
It has to be mainly an issue with budgets. You'll also notice how the same droid props keep showing up over and over again. As far as aliens go, the Rebels had Chewie and the empire had that Kuboz spy that ratted out Han on Tatooine. The attack on the Jawas seems to imply that the sand people were working with the Empire (otherwise, how did they get ghaffi sticks and make bantha tracks?)
I think the whole "Empire is human only" thing started in ESB with the "we don't need that scum" line about bounty hunters. I'm not a big fan of that entire shift in the way the Empire is presented. I'd rather they were evil because of their totalitarian ideology than they are evil just because they're racists.
[Humans] simply never encountered any other species that had developed a hyperdrive.
My idea is somewhat different. Consider all the weirdos in the cantina. Ben said that the best starpilots hung out in there. My impression is that several of those weirdos were starpilots. Tatooine ("If there's a bright center of the galaxy, you're on the planet that it's farthest from.") I take to be on the edge of the Empire, and therefore bordering non-human space. As such it would be a natural pit stop for starpilots of various races.