Because Hades had kidnapped her there and she hadn't wanted to move there. Also, the Underworld is a terribly sad place with all the souls, lost dreams and hopes, and all those other things.
Persephone likely took it to be a solemn duty as Queen of the Underworld, but not without love and joy.
Hades took Persephone to the underworld because of Persephone's beauty.
Queen Persephone is the queen of the Underworld. Hades is her husband and is ruler of the Underworld.
Persephone leaves the underworld in Spring and she returns in the beginning of Fall.
Persephone is both the goddess of Spring and the Queen of the Underworld.
It is not described in myth.
Hades and Persephone had a palace in the Underworld.
Hades took Persephone to the Underworld to be His wife. Zeus (Persephone's father) promised Persephone to Hades as his wife.
Persephone ate of the pomegranate in the Underworld and so had to return to the Underworld every year.
Persephone was not Queen of the Underworld until taken to the Underworld by Hades: afterward, she was never taken anywhere against her will.
the underworld
Persephone is a Goddess of the Earth, and the Goddess of the Underworld.