George Lucas may not have forgotten about Obi Wan serving Bail Organa… he probably just didn’t think that one throwaway line, written at a time when the Jedi backstory was hazy at best, fit with his evolving conception of the Jedi. In fact, that line about Bail Organa is a vestigial remnant of Lucas’s original conception of the Jedi.
That line actually helped form my conception of what the Jedi were like, before the Prequel films came out. But the Jedi turned out to be very different from how I imagined them. That line about serving Leia’s father (Bail Organa), along with Obi-Wan’s recollections to Luke, the use of the word “crusade”, and Kurosawa’s influence on Lucas, suggested to me a Jedi order that was more like some combination of Medieval Knights and Samurai, who served the nobility during the Old Republic. I imagined Obi Wan was in service to the royal family of Alderaan. But I certainly never imagined the Jedi were actually more like celibate Franciscan monks.
The scarce info we get about the Jedi in the OT suggested to me that the Prequel era was something closer to Knights of the Old Republic, with the “high fantasy” element cranked up a bit more than was evident in the militaristic OT era, and the Jedi Knights existing less as a monastic order and more like an elite class of warriors associated with the nobility.
It turns out my conception of the Jedi was very close to the way Lucas originally envisioned them - he imagined them as the personal bodyguards of the emperor, and as an elite group of warriors who were killed off by a rival warrior sect, the “Knights of Sith”. (This is how Lucas describes them in the 1974 draft.) Obviously, Lucas’ conception of the Jedi changed drastically over time. He ultimately made them less “Knights of the Round Table/Samurai” and more “celibate Franciscan monk” for some reason. But that line about Bail Organa is a vestigial remnant of Lucas’s original conception.