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In Wookieepedia it states that humans (the race that originated from Coruscant) were favored over alien species. The Rebellion and later on The Resistance seems to use more aliens , particularly in the upper ranks.

Many humans changed the course of history, from politicians to a mere slave boy turned Jedi Knight.Sheev Palpatine, who founded the autocratic Galactic Empire that ruled over the galaxy for two decades, was a human from Naboo. Many members of the Alliance to Restore the Republic's leading circle, including Mon Mothma, Bail Organa, and his adopted daughter Leia Organa were humans as well. The Galactic Empire was known to favor humans at the expense of alien species. The Empire's government and Imperial Military was dominated by humans.

And the Imperial Military from Wookieepedia.

Enlistment was open to human men and women, who were trained at the numerous Imperial Academy complexes across the galaxy, with many members becoming enlisted personnel, officers or Stormtroopers. Since the Empire favored humans and looked down on Aliens, it was rare to find non-humans serving in the Imperial Military

Is their anything canonical that states why there usage was prefered?

  • in the 4, 5, and 6th movie the imperial army is all clones of the one human, so by general assumption you can say that it was just easy for them to use clones, – Fox-Chan May 17 '16 at 15:12
  • @Fox-Chan That's not true. The stormtroopers were volunteers. – Null May 17 '16 at 15:15
  • so did they kill all of the clones then? – Fox-Chan May 17 '16 at 15:17
  • @Fox-Chan No, they were phased out of the Imperial military. They aged twice as quickly as normal, so that happened pretty quickly. – Null May 17 '16 at 15:19
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    There are lots of relevant questions/answers to this. This one provides a Legends explanation. This one talks about the First Order. This one discusses the Emperor. – Null May 17 '16 at 15:21