I believe that Star Wars did set the stage for reviving the fantasy genre. And I'm sure that part of its appeal was its "breath of fresh air" factor from the times. But the fact of the matter is that it was simply a good enough movie to do so. Star Wars wasn't the only science fiction/fantasy movie to come out in that time period, so it wasn't simply because Vietnam was over, and we needed something new. If some other like movie had revolutionized a year or two before Star Wars, Star Wars still probably would have been huge because it was just good.