Because if you were no longer able to take part in Church services, confess and receive forgiveness etc., you were done for. I haven't heard that it doesn't, but someone on here may know.
It is a public proclamation of the Church that a member has spoken or acted in such a way as to put the church community in bad light. Excommunication is the Church's way of saying the 'offender' is talking *** and/or is not representitive of the Church's doctrine.
If this still applied today in it's intended purpose there would be a noticable reduction in the number of christian dignitaries.
Our perception of it seems to be somewhat dramatised with high profile cases but I believe it was a widely used sanction and could be and was regularly forgiven after a repentance.
The old image of "Bell book and candle" was somewhat over over dramatic, although I guess the belief was that you went to hell if you died whilst under excommunication.
Of course one of the most dramatic examples was King John's row with the papacy which resulted in the whole of England being placed under interdict between 1208 and 1213 which was a sort of national excommunication
In days of yore all rites of passage were held in a church context. With reference to the Interdict during the reighn of John, I remember the cod history of England by Sellars and Yeatman, called "1066 and all that". They said that during that time nobody was allowed to be born, marry or die.
It does still exist today, but I don't think it carries the weight it did when the Church of Rome ruled all. The most notable case in England's history has to be that of King Henry VIII.
Here's a health to the Protestant Minister
And his church without meaning or faith
For the foundation stones of his temple
are the bollix of Henry the Eight
A sad day for Christendom the day that scoundrel was born...
I think a fairly high profile case is Bishop Richard Williams, who was excommunicated by Pope John Paul II for proclaiming that the Holocaust didn't really, happen, and it was all Jewish propaganda.
He was reinstated by the current pope, Benedict, who evidently feels that Bishop Williams' views are not, after all, inconsistent with institutional Catholicism !!!!!!!
Was he not excommunicated because he refused to say the Mass other than in Latin? The question of the Holocaust was a minor matter in the eyes of Mother Church
If only he had followed his faith . I think he would have made a cracking Christian Brother teaching in some Austrian school. The world would have been a much better place