Why was Medusa's hair made of snakes?
It was an especially cruel and specific punishment for Medusa as she was known for her beautiful golden hair
Medusa's transformation from a beautiful golden haired priestess of Athena to not-so-beautiful Gorgon snake-lady was not instant, is was a gradual and drawn out punishment:
She was originally a golden-haired, fair maiden, who, as a priestess of Athena, was devoted to a life of celibacy; however, after being wooed by Poseidon and falling for him, she forgot her vows and married him. For this offence, she was punished by the goddess in a most terrible manner. Each wavy lock of the beautiful hair that had charmed her husband was changed into a venomous snake;