Why was the scope of Synod 16 limited to evangelization?

Blessed Paul VI called evangelization “the grace and vocation proper to the Church, her deepest identity. She exists in order to evangelize, that is to say, in order to preach and teach, to be the channel of the gift of grace, to reconcile sinners with God, and to perpetuate Christ's sacrifice in the Mass, which is the memorial of His death and glorious resurrection.” Evangelizing, spreading the Good News, proclaiming the Kerygma… are central to all that the Church does.