The official novelisation for Attack of the Clones gives the impression that about five years before the events of Phantom Menace there was an almighty brouhaha in the Jedi Council. The upshot was that the moderates won the day and, as a result, Sifo-Dyas was forced to step down, Qui-Gon Jinn was refused a seat despite his obvious credentials and Count Dooku left the order entirely over much the same issue, that the Jedi Order was serving a corrupt Republic.
Some time before this period, Nu (who apparently had sympathies for all three of these individuals and their argument) left the Council so that she could work as Chief Archivist, a role that seems to be little more than a sinecure.
No specific reason is given for her depature, but it would appear that those with strongly held views about opposing, or at least restraining the excesses of a failing Republic have been systematically moved into less powerful positions while those who support the status quo have risen in power.
“Well, one might say he [Dooku] was a bit out of step with the decisions of
the Council,” the Archivist replied. “Much like your old Master,
“Really?” Obi-Wan prompted, wanting the information about Dooku, of
course, but also hoping to garner some insight into his old and
“Oh, yes, they were alike in many ways. Very individual thinkers.
Idealists.” She stared at the bust intently, and it seemed to Obi-Wan
as if she had suddenly gone far, far away. “He was always striving to
become a more powerful Jedi. He wanted to be the best. With a
lightsaber, in the old style of fencing, he had no match. His
knowledge of the Force was… unique. In the end, I think he left
because he lost faith in the Republic. He believed that politics were
Jocasta Nu paused for a moment and looked at Obi-Wan, a very revealing
expression that showed she did not think Dooku as out of step as many
of the others apparently did.
“And he felt that the Jedi betrayed themselves by serving the
politicians,” the Archivist stated.
Obi-Wan blinked, soaking in the words. He knew that many, Qui-Gon
included-even himself included, at times-often felt the same way.
Star Wars: Attack of the Clones - Official Novelisation