TheBoost said:

I'd lean more towards budget limitiations, and simple expediency.

In Star Wars, unless being an alien was a important point (scary bars, Jawas, cool co-pilot) human was the de facto species just for simplicities sake.

I think that this is exactly it. The first film was greatly limited in its representation of non-human species because the film makers had to rely on cheap alien designs and off the shelf rubber masks. The cantina scene got away with a lot by having the creatures in the dark recesses of the set. Still, Lucas and others were so unsatisfied with the results, they got extra money to build more articulate masks for a few creatures like Greedo.

Empire re-used a lot of non-human costumes fron Star Wars, but it seems that the Empire and the Rebellion against the Empire were really items that primarily concerned humans. The Empire (I am sure that there is some EU explanation for this) ended up being composed of human participants and the Rebellion consisted of humans who belonged to the Empire but left. If the Empire was mostly human to begin with, the exodus from and rebellion against would be, at least initially, mostly human as well. It is not a matter of non-humans not being involved in the Galactic Empire or being affected by its tyrannical grip on the galaxy; but rather an Empire and Rebellion consisting of mostly humans and droids that ended up assimilating other races and species as time went on.

So non-humans really aren't in the movies because 1. until ROTJ, they were just too much of a pain in the butt to make convincingly, and because of this 2. Lucas made the Empire and the Rebellion mostly human entities with a few droids.