IIRC George Lucas had the entire "story" more or less all mapped out before he started ANH. Mind you not concretely. My guess would be that after Lucas' death Star Wars remains a stand alone film well into the 1990's. Star Wars is big, but nowhere near as big as OTL. Finally in the early 1990's one of Lucas' relatives is pouring over old Uncle George's files when he discovers the overall plotline for Star Wars. He then goes and sells said plotline to Universal or Warner Bros. or some company like that. In order to appease the fairly large fanbase that accompanies the Starwars name, the writers decide to stay as close to the script as possible. In an effort to promote continuity three Star-wars films are planned and filmed consecutively. Starwars IV, V, and VI arrive in theaters in 2001, 2002, and 2003 respectively. The prequels meanwhile are turned into a highly profitable TV series lasting 8 seasons and succeeded by yet another series chronicling the events after the trilogy.