For more or less the same reason Hades himself was: it was his job. If there's a river that you can't Ford or swim across, there has to be a ferryman. Charon was that ferryman.
Charon, the ferryman of Hades
Hades has a boat full of Charon.
Hades is the ruler of the Underworld. Charon (not a god) ferries the dead across the river Styx into the Underworld.
Charon.
His name is Charon.
Charon, The Ferryman.
Charon was the ferryman of Hades.
The ferryman in the Underworld is known as Charon.
Charon
Charon .
percy bribed charon and promised to ask hades about a raise in his pay
Charon was not a Greek god- he was Hades' ferryman down the river Styx. He did not die.