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?