Dear Dagmar,

I did a little digging on the Internet and found some articles that talk about this topic. They could do a lot better job that I could because the Traditional Mass is quite a bit before my time.  :-)

In the meantime, here is the reason that the Council of Trent gives:

And since the nature of man is such that he cannot without external means be raised easily to meditation on divine things, holy mother Church has instituted certain rites, namely, that some things in the mass be pronounced in a low tone and others in a louder tone. She has likewise, in accordance with apostolic discipline and tradition, made use of ceremonies, such as mystical blessings, lights, incense, vestments, and many other things of this kind, whereby both the majesty of so great a sacrifice might be emphasized and the minds of the faithful excited by those visible signs of religion and piety to the contemplation of those most sublime things which are hidden in this sacrifice.  (Session XXII, Doctrine Concerning The Sacrifice Of The Mass, Chapter V)

I think that it's important to remember that the Holy Mass is not an "American Invention".  Many people expect the Liturgy to reflect exactly the culture that they and they alone live in...  that is the reason why traditional Masses are frowned upon so much these days in The United States and that "Guitar" Masses seem to be embraced.  People don't seem to realize that the Holy Mass was celebrated long before the New World was even discovered.

But actually the Tridentine form of the Mass originated in Europe centuries ago where silence was considered much more reverent than it is here in the US.  I believe it is still true today in Europe... I have never been there personally but friends have told me that the celebration of the Mass in Europe is MUCH less of a "public relations" event than it is here in the US..  many times you can't hear what's going on at all and that's still considered "normal".  Yet here in the US it is common to see cry rooms, camera's and TV's set up right in the church, audio aids, etc.

Getting back to the time of Trent, back in that culture, it made perfect sense to have the priest say the mass inaudibly since the prayers of the Mass are mostly his prayers alone, which he offers on behalf of the people, but not necessarily along out loud WITH them.

Getting back to the above cite from Trent, it seems that the Church was saying silence makes it easier to meditate on heaven on Earth, which is what the Mass is.  All the visuals and smells also help add to the "majesty of the sacrifice", and to the sublime things hidden inside.

It should be noted that there actually ARE responses in the low Tridentine Mass, the servers reply on behalf of the people who may or may not be able to follow along in Latin.

The best way to answer your question about why the change to the spoken Mass with the people would be to direct you right to Vatican II, to the document Sacrosanctum Concilium which goes into this issue in detail.  Concerning the reformation The Council directs that a full active part in liturgical celebration is "demanded by the very nature of the Liturgy" and it is of "paramount concern, for it is the primary, indeed the indispensable source from which the faithful are to derive the true Christian spirit"  (see n. 14)

Quite honestly I agree that active participation is demanded nature of the Liturgy" since the Liturgy is the universal prayer of the entire Church, not just the priests.  So to me it makes perfect sense to have the people praying out loud at the appropriate times with the priest.  As for the reason why the change only occured recently, I think that can be greatly debated, however I'm sure that it would have something to do with the changing culture.

Finally, about the few parts that are said out loud in the traditional form.  As far as I know these were originally prayed out loud mostly for practical reasons..  so that the servers, people and choir would be able to follow along.  Over the centuries these practices may have been retained simply because of the tradition.

Mr. Slavek