Star Wars was originally a single film. A New Hope was just that, a one-off film meant to tell a single story. According to rumors, A New Hope was tentatively called Star Wars: The Adventures of Luke Skywalker. Lending credence to the fact that A New Hope was to be a single film and not the first entry in the series, is the fact that early versions of the film shown in cinemas did not have the Episode IV moniker on the title screen during the opening crawl.
It wasn't until later that Lucas decided to extend the story, partly because of the financial success of the movie and also partly due to demand by the fans.
As such, Lucas chose to focus on the characters people were already familiar with, instead of making up brand new ones.