Because Latin is the perpetual language of Holy Church, and because ever since Our Lady gave us the Rosary some 8 centuries ago that is how the saints have prayed it.
Is it less good to pray in the vernacular? No, it is perfectly fine, and it is good to know the prayers in one’s mother tongue.
But Latin is, as Pope John XXIII states, "noble, majestic, and non-vernacular…truly catholic…consecrated through constant use by the Apostolic See, the mother and teacher of all Churches… esteemed “a treasure … of incomparable worth”.
You will understand more the fundamental importance of Latin by reading the Apostolic Constitution
Veterum Sapientia of Blessed John XXIII (
English translation) as well as the Apostolic Letter
Latina Lingua of Pope Benedict XVI.