King Sha'ul failed because the Divine Will was too much opposed to his human inclinations. Because of the failure, David had to fail, and Yosef (the ten tribes) and Yehudah had to separate. Yosef represents freedom. Yehudah represents servitude. The contradiction proved to be unbridgeable.
Conversely, the acceptance of Torah in the time of Achashveros healed the fundamental reason for the division. Indeed, we see that the book of Esther hints at this Tikkun. Mordechai is from Binyamin, ben Shim'i ben Kish. Why is Kish mentioned? King Sha'ul was also ben Kish, and also from Binyamin. Mordechai and Esther succeeded precisely and emphatically where Sha'ul failed. Where Sha'ul failed by not killing Agag, the Jews under Mordechai and Esther killed B'nei Haman with all other Amalekites. Where Sha'ul failed by taking spoils of war, the Jews under Mordechai and Esther did not take spoils (Esther 9).
Freedom and servitude to HaShem unite when human will is in line with Divine Will. This is the level of Binyamin ben Ya'akov. This is hard ro reach. This means crossing the Sambatyon. But there lies the secret of the unison of Yosef and Yehudah. This is the repair of the Sin of Adam HaRishon.
צריך לדעת איך עיקר הכוונה בבריאה היה רק להוציא האדם שיהיה בעל בחירה... ועל כן כל הבריאה נמשכה למה שצריך לזה הענין
One needs to know how the main purpose of Creation was purely to produce man such that he has free will... and hence Creation was constrained by what was needed for that.- Iggrot Ramchal 14 (appears before the segment translated in the above link).
