Thanks to the decades of lore that have crafted Star Wars, even the smallest details are given an explanation of how they connect larger events in the galaxy. However, at the end of the day, there are more questions than ever about why, and how, certain events unfolded the way they did. One of the biggest questions fans have had is why 1977's Star Wars was called Episode IV, despite being the first film in what would become a blockbuster franchise.
Source：https://www.cbr.com/why-george-lucas-started-star-wars-with-episode-4/
