I was just thinking a minute ago ( I try to do this at least a few minutes a day), when I realised that there were not any aliens in the Empire. What up wit dat? They occasionally dealt with aliens, the Hutts, bounty hunters, etc. but as far as I could tell there were no aliens actually amoung the ranks of Empire (Hell, they’re really were not any amoung the rebellion untill the last movie.).
So, why?
Because the Emperor was a vile racist. That’s pretty much the only explanation that’s been given so far.
There were, of course, aliens who happened to live in the Empire - residents of conquered planets and such who were relegated to non-citizen status, but none in the military or government. The exception to the rule (so far) would be Thrawn, but only because he was both very very useful and wasn’t too freaky looking.
The real reasons are that it’s cheaper to have humans than aliens and that humans in uniforms are easier to make look serious and sinister than aliens in anything.
The fluffy reason was addressed in some of the books, and was spelled out as the Emperor’s NHM (NonHuMan) policy. Basically, he didn’t like aliens, he didn’t like women. There were a few of both, but they were rare.
I find it hard to believe that the emperor, while being very evil, could be this stupid. Also, did not the dark lord train one Darth Maul. Unless that boy had some serious body mods, I do not think that he was human.