There was a lot of skepticism at the time of the release of the first movie - remember, it initially had a very limited release. The movie had gone over budget, taken longer than had been allotted for filming and a lot of the cast (especially Harrison Ford and Alec Guiness) didn't buy into the premise at all, or have much faith in it. Ford basically thought he was just making a kids movie.

Further to that at the time there wasn't really that large a market for movies as fantastical as Star Wars - in the post Vietnam War era gritty grungy dark realistic movies were very much par for the course.