While I am unaware of a directive from the Church stating that one may not pray the rosary during Mass, the Church usually does not legislate on what the laity is not supposed to do at Mass. It lays out what the laity is supposed to do and assumes that the laity is doing it.
While praying the rosary during Mass is a custom from the time when the Mass was in Latin and people had a more difficult time following along with the liturgy, the laity was never supposed to be praying private devotions during the public prayer of the Church. The rosary is a private devotion and, while good in itself in its proper setting, should not take the place of or distract us from our assistance at the Mass.
