Because she "falsely" obtained it and was not done in trial by combat, the planet Mandalore was cursed and ultimately ravaged by the Empire in a Night of 1,000 Tears. She believes it was because of her own wrong doings that despair fell upon her planet. Now she seeks to properly obtain the weapon in trial by combat and restore the planet and unite the Mandalorians once again.
Because in that scene all of the mandolorians were bowing down after saying they would serve under her and say they believe she should lead . In that moment when all the clans are saying they believe she should rule she accepts . This dosent go well and she ends up loosing it believing it was because she didn't win it in combat she trys to re obtain it but in the correct way so that she dosent loose it again . To put is simply she made a mistake and when she lost it she made sure to get it the right way the next time
