Why can Demeter only spend 6 months per year in earth?

It is Persephone, Demeter's daughter, who spends half of the year in Hades and half of the year with her mother. According to Greek mythology, Persephone (pronounced per-seff-oh-nee) is abducted and carried off to the underworld by the god of the underworld, Hades. While there, she eats pomegranate seeds, which bind her to remain with Hades for at least part of the year. In the spring and summer, when everything is new and blossoming, Demeter is expressing joy that her daughter is returned to her. In the fall and winter, when things die or go dormant, Demeter is grieving that she must be departed from her daughter.