The old gods were given the ultimatum of becoming guardian angels or fragile mortals. Most gods chose to turn into guardian angels, including most of Artemis' siblings. Some were even thriving in their new roles like her vibrant twin brother Apollo.
Artemis didn't fear mortality like her fellow Olympians. She actually envied the mortals and their way of life. They seemed to enjoy life differently from us gods. They loved, laughed, and cried differently.
In the end, Artemis made the unthinkable decision and became a mortal, living among the children she would have been looking over as a guardian angel.