the reason he hadn't labeled ANH episode IV was 'cause he didn't think it would do all that well in theaters and since it did, he retitled it a new hope. Fox was on his crack about finishing or even having the idea of a star wars movie. so george hid TESB and ROTJ.
Originally it was one big movie, but the script was a little long and had too many gaps in it. again, i like having a firm grip on the timeframe of when events occured.