A spiritual setting affords them the opportunity to do just as they please. Unless the pastor is exceptionally strong, he's no match for a narcissist determined to get his or her own way. It also has to be tough, from a pastor's perspective. His job is to save souls, and not making people feel welcome in his parish runs counter to that.
It's a tough balancing act. As followers of Christ, we are supposed to not judge anyone's interior motives. We're supposed to put up with people's personality quirks, and to see the good in them. However, there does come a point where bad behavior and unkind gossip can cause a lot of disruption.
Narcissistic types also sow a lot of confusion wherever they travel. So it's typically difficult for anyone (pastor included) to discern just where the chaos originates. Oftentimes the wrong person is blamed for the trouble, and this is the party that ends up leaving a particular parish.
The same dynamics that can play out in an office, or in a social group, can happen in church if a malignant narcissist senses a weak spot and gains a position of power. Except that, in church, he or she can get away with a lot more than they would anywhere else.