The way I see it as an adult, the apparent absence of aliens is obviously down to budgetary and logistical restraints.
As a child I'm not sure it really crossed my mind until ROTJ when I just figured that Mon Mothma had united all these splinter groups of various species rebels (including the humans).
Even in ROTJ, I think the only aliens we see involved in the Endor assault and Death Star attack are chewbacca, nien numb, and a pruneface gets on the shuttle never to appear again. I don't recall any aliens flying the A wings, etc. You only see them in the briefing room and the rest of their involvement is implied? Oh, I suppose the ewoks are aliens, too ...