Question: why did mipha have to die?
Answer: During the Great Calamity, Mipha succumbed to the Scourge of Divine Beast Vah Ruta, Waterblight Ganon, which then possessed the Divine Beast. After Link defeats Waterblight Ganon and frees Ruta, Mipha greets the hero, stating that because of his courage she and Vah Ruta are now free.