When a deacon is present at the Celebration of the Eucharist, he has some special duties that he is responsible for. These duties include proclaiming the Gospel (and possibly the homily), praying the General Intercessions, assisting in the distribution of Communion, and carrying out other liturgical duties as necessary. So although at World Youth Day there were tons of priests that could have proclaimed the Gospel, it is the rightful duty of the deacon to do so, if there is a deacon present at the Mass.