Because
1. He had no choice and
2. He landed on her island 9 days after Charibdis broke up his ship and killed his men
Calypso wants Odysseus to stay with her on the island forever.
Odysseus met Calypso on her lush island of Ogygia.
Calypso
She loves Odysseus and wants to keep him on her island, but Zeus wants her to let Odysseus go.
Calypso
Calypso is the last one to keep Odysseus against his will on her island. The name of the island Calypso kept Odysseus on was Ogygia.
Calypso offers to make Odysseus immortal and forever young if he will stay with her on her island. Odysseus respectfully refuses her offer.
Calypso.
Calypso does.
Calypso
Hermes tells Calypso to let Odysseus go.
The god holding Odysseus on the island is Zeus. Zeus got angry at Odysseus and his men for eating the CATTLE of the sun god, Helios. Odysseus didn't eat the cattle, though. He survived, but his men (or crew) didn't. They died. They Odysseus floated to Calypso's island, Ogygia. Calypso loved Odysseus. They got a little "busy", but Odysseus still loved Penelope. Athena begged Zeus to make Calypso release Odysseus, since Calypso was trapping Odysseus on Ogygia. Zeus told Hermes (god messenger) to tell Calypso to let Odysseus go. She got the message, and agreed to release Odysseus. Odysseus built a raft, but while traveling, Poseidon destroyed it, since Odysseus blinded his son, the Cyclops, Polyphemus. For more information, GOOGLE works GREAT!!! Hope I helped! (: