The ostiarius (or door keeper) evolved after the legalization of the church from existing Roman customs. While initially, (under church persecution) there was a need for protection from persecution, this did not become an official title until well after that need. It became fashionable, however, to adopt the roman custom. Just about any good house would have had a "bouncer" so to speak. As the church grew in the Middle Ages, and by fiat the entire populace was assumed to be Christian, the role itself became vestigal.
The death knell for the ostiarious was Vatcian II.
To wit, Wikipedia says:
The porter was not a part of Holy Orders administering sacraments but simply a preparatory job on the way to the Major orders: subdiaconate (until its suppression, after the Second Vatican Council by Paul VI), diaconate and the priesthood. Like the other minor orders and the subdiaconate, it is retained in Indult Catholic societies such as the Priestly Fraternity of St. Peter.
In general, Vatican II is often hitched with a view towards opening the doors to the common man. The early church was a very open and flexible thing - Paul, for example, would say that he became "all things to all men so that by any means I might save some." Vatican II re-emphasized this, seeking to knock down all barriers that kept people from Jesus. Saying the mass in the venacular (as opposed to Latin), for example, was designed explicitly to knock down a barrier. The ostiarious was literally one such symbol.
In general, the attitude of what the church is has shifted over the centuries. In the Middle Ages, it was a powerful thing, and by its own admission, a self-selected elitist club.
The modernization and secularization of the culture has made this stance impossible to hold. Like with excommunication, adding a bar to an voluntary organization is inherently counter-productive, but also not the point of the church in any event. Today, the metaphor now is more akin to the church as a hospital for wounded sinners. All of this openness is, for many, simply a return to the model of the early church, and hence a Good Thing(TM).