Why did Hylia become mortal?

Why did Hylia become mortal? In addition to what tlozbj said, Hylia had to become a mortal because she suffered grave injuries after the battle with Demise. It was either become a mortal with a plan to save the world, or simply die and let the world get destroyed once Demise broke free.

In addition to what tlozbj said, Hylia had to become a mortal because she suffered grave injuries after the battle with Demise. It was either become a mortal with a plan to save the world, or simply die and let the world get destroyed once Demise broke free.