Palpatine was always portrayed as racist: he hated aliens, perhaps believing in superiority of the "master human race" (whateveryacallit), that's why the Empire was enslaving aliens from many worlds - take Wookies, for example. They were used for hard manual labor. Wasn't Ackbar Tarkin's slave?
The Empire is modelled in the movies to give us the "look&feel" of our real-world racist (hate-oriented or whatever you call it in PC lingo) systems/organizations. So you easily believe in racist Galactic Empire, because in its military ranks you see only white human males. While geeks may speculate on "what lies beneath stormtrooper's helmet" giving examples from EU, let's face it - this is how it was made for the movie and the effect is working. The contrast is well seen when compared to egalitarian Rebellion (that's how we call the Alliance
), where all not-necessarily-white men, women - and even non-humans! - are welcome.It would give us a nice continuity - and again nice effect of contrast - if we could see that in the PT world even more crowded with alien races (see: the senate) Palpatine plots his dark deeds only with (white) humans. Because those pale ill-looking Imperial Officials were indeed creepy and sinister.
GL could use alien Palpatine aides on the condition of showing us in the end that they're expendable for Palpatine. So e.g. Maul could die because Palpatine intended him to perish in battle (to "free the slot" for new apprentice).
Instead we got a bunch of new costumes and CGI. Why? Is there a deeper meaning? No. "We added them because they look cool, aren't they? Look what we can do with our rendering farms!"