Because the goal of the Church is to help the sinner reform and repent, not ‘use’ the sinner to motivate others. The idea of ‘making an example’ is based on the concept that some people are expendable.
We hold that each person, regardless of stage of development or condition, is a unique creation, infinitely loved by God. If we reject that concept for the sake of disciplinary example, we undermine the moral foundation of important teachings, like abortion.
Put it this way, if you attend Mass regularly, you are asked to call to mind your own unworthiness. Have you ever had a week where you thought to yourself, ‘God, I’ve been perfect this week, so why don’t we just put this one in the bank in case I slip next week…’? If not, why shouldn’t the Church not excommunicate you? Week after week, year after year, and you are still a sinner…
Seperating ourselves (‘well at least I never had an abortion…’) is moral relativism and we are warned expressly not to make such comparisons in the Gospels. Also, we are not so seperate from our fellow man as we sometimes think - if the abortion was an act of desperation, and that desperation is something we, ourselves, could have helped lessen, are we not also culpible? Think of the reading we had last year, with the wealthy man at his table, and the wretch at his door. The wealthy man did not actively sin against the wretch, but he faced damnation for indifference and inaction.