Answer
Natural selection tends to prevent hybrid formation, which would waste work since it mixes species. Once hybridization has occurred, the resources spent would be wasted if postzygotic mechanisms were selected for. To avoid this waste, postzygotic mechanisms are not naturally selected.
Work Step by Step
First consider the nature of natural selection, and then consider when postzygotic mechanisms act. Think about the lost resources, and you can see why only prezygotic mechanisms would arise via natural selection.
