Before Star Wars most sci-fi films were a bit of a joke. Dodgy special effects, spaceships made out of plastic or cardboard, and monsters that were obviously humans dressed up in badly made costumes.
Lucas had had a hit with American Graffitti but when Lucas tried to get the money to make Star Wars he met with little enthusiam from the studios (sci-fi films had never made money and were not taken very seriously).
So he had enough trouble making one Star Wars movie, without thinking about it becoming a series (also at that time making "follow on" films was not that common).
It was only when the first Star Wars film came out, and was such a massive hit, that the film industry (particulalry with the add-on merchandising) became interested.
So the next two films followed on from the first, until much later they decided to do three prequel films.
As any "Star Wars" film was always likely to make money Lucas could probably have done anything he liked but decided to do prequels.