Taking place on a planet of Wookies would make the Empire losing the ground battle make significantly more sense. Legend has it that it was changed to Ewoks for the kid toy opportunities.
I don't doubt that for a moment, but I think I once heard an interview with Lucas where he explained that, by the time they got to the third movie, Chewbacca had 'become' more intelligent, and that he specifically wanted the empire to be defeated by a 'primitive' people.