I read somewhere that Lucas wanted to be a race car driver - hence the speeding tickets that kept him out of the Air Force...
Also, back in the late 60s and early 70s Hollywood basically gave anyone with enough guts, guile and talent a chance to make a movie. The doors were suddenly open for a brief little while. The only requirement was that the director was young because back then the executives had absolutely no idea what the baby boomers wanted in their films, so hits like Bonnie and Clyde, the Godfather and American Graffiti were huge surprises to the studio execs - many of whom were themselves very old and out of touch (some things never change).
It was a massive untapped market that got tapped by other boomers like Lucas, Coppola, Spielberg, Scorsese, etc...
My point is that Lucas, no matter how weird his films would have been would have gotten a shot or two or three simply because of his youth and association with Francis Ford Coppola. Back before THX 1138 he was a sort of apprentice to Francis and he worked on The Rain People with him. Coppola made the Rain People and two other small films that made very little money before he was given the Godfather, and that happened mostly because of his Oscar for writing the screenplay for Patton. If the Godfather weren't a hit then Francis would never have directed another big budget film again. Coppola even toyed with the idea of writing the screenplay with Mario Puzo and giving it to Lucas to direct. Could you imagine that? I shudder to think...
Another thing - Lucas was never a people person. He often hired acting coaches to deal with the actors so making a character driven film like Graffiti was very difficult for the wallflower Lucas, so maybe a few tours in Vietnam would have brought him out of this shyness...
Personally, of all the great auteur directors of the 70s that the United States produced, Lucas is my least favorite. To me he is and was barely a director who never really wanted to be a classical director and who would much rather work with scale models and computer animations than with people. He is a born producer/special effects person not a director. He never even directed the best Star Wars film - the Empire Strikes Back. Star Wars was a good premise, a cute story, but in the end way too gimmicky for me and no where near dark enough.