Learning the reason is tied to an unforeseen power that allowed him to survive the effects of Auswählen the Yhwach used to kill all impure Quincys to gain their power, Uryū accompanies Yhwach to the Soul Society.
Learning the reason is tied to an unforeseen power, Uryū accompanies Yhwach to the Soul Society. This unforeseen power allowed him to survive the effects of Auswählen the Yhwach. This Auswählen Quincys to gain their power. The Yhwach was used to kill all impure This Auswählen.