The United States Conference of Catholic Bishops does not have any authority to excommunicate.
The only one with authority to excommunicate him is Archbishop Wilton Gregory, the Archbishop of Washington D.C. He is known not to be inclined to take action in this regard, though canonically if he were inclined, he could.
Unfortunately he, and many bishops in the United States Church, are not opting to take a firmer stand on this.
In the end they will be called to account for this, as Biden will.