If we are incorporated into the Church by sanctifying grace, why are we not excommunicated when we commit a mortal sin?
One who commits a mortal sin does sever himself from Our Lord - until he repents.
If you’re asking why the Church doesn’t declare the person excommunicated, a couple obvious reasons come to mind:
- how do we know whether the person has sinned mortally, losing the life of sanctifrying grace (only God can judge the heart of the sinner to know his degree of guilt);
- even assuming it was a true mortal sin, how would one know that the sinner hadn’t repented (unless they made some sort of public declaration saying they had no intention of repenting)
The Church is usually very slow to publically excommunicate someone.