The story goes George wanted the IV in the 1977 crawl, but Fox told him nobody would understand why the first movie in the series was actually the fourth. So he relented and just had “Star Wars” instead of “Episode IV A NEW HOPE”.
I read somewhere that there was a belief by George and/or the studio that the film might not do very well and would be the one and only Star Wars film; so it was only called “Star Wars”.