Because he outlined the story from the events of Phantom Menace all the way to Return of the Jedi before the first movie even went into production.
Then he decided to tell the second part of the story first, THEN realized the script was too long for even that and split it into three movies.
He wasn’t sure he’d be able to commit the whole story to film, so he thought the introduction of Vader’s son and the destruction of the super weapon was the most important part and was able to stand up on its own.
When he realized he could actually finish the second act of the story, he surprised the viewing audience by calling Empire Strikes Back “Episode V” and retroactively calling Star Wars “Episode IV A New Hope”.