Catholics: Why hasn't Pope Benedict XVI excommunicated Pro-Choice Politicians from the church?

I know that some pro-choice politicians are denied communion as long as they are pro-choice, but I think there is also an option to excommunicate, although it doesn't seem like the Catholic leadership is willing to go that far. Just remember that the areas of the country that are the most Catholic in representation are also the most liberal and pro-abortion. I'm sure some Catholic leaders must be in the pockets of some politicians.