An excellent question! I posted the following historicial perspective to a different thread yesterday. In case anyone here missed it, it's worth repeating:
"Pius V did indeed do something unprecedented in the history of the Church, and that was to freeze, in effect, the liturgy as it had developed up to that point in the 16th century. In doing this, Pius V stabilized, homogenized and thus preserved the unity of the Church during a turbulent time of mass apostasy by preserving the venerable and sacred liturgy of the centuries. His act, unprecedented though it was, was yet done out of a perceived necessity given the Protestant Revolution."
"We must not have the simplistic view, as some of our badly misinformed traditionalist brethren, that the Novus Ordo Mass is intrinsically evil, or invalid. Michael Davies, a very renowned scholar from England, has, in many of his works, sufficiently shown that this could not be true, that the Novus Ordo is somehow intrinsically evil. In his various works on the Mass, he has demonstrated, in fact, the intrinsic goodness of the normative Mass, published by Paul VI, not to be confused mind you, with any wretched translations that may have appeared in the vernacular, especially the English language. "
"On the other hand, there must be said something for the defense of the authority of the Catholic Church, which has the power to guard and protect the sacred liturgy. "
"We must not have the simplistic view, as some of our badly misinformed traditionalist brethren, that the Novus Ordo Mass is intrinsically evil, or invalid. Michael Davies, a very renowned scholar from England, has, in many of his works, sufficiently shown that this could not be true, that the Novus Ordo is somehow intrinsically evil. In his various works on the Mass, he has demonstrated, in fact, the intrinsic goodness of the normative Mass, published by Paul VI, not to be confused mind you, with any wretched translations that may have appeared in the vernacular, especially the English language. "
"So what were some of the changes that were needed, according to the Council Fathers? Sometimes in their fervor for promoting the traditional Mass, traditionalists blind themselves by the following erroneous propositions: (1) to cast the blame entirely on the twenty first ecumenical Council of Vatican II as the cause of all the invalid, sacrilegious and innovative liturgical experimentation of today. Vatican 11, albeit a pastoral Council, was still protected by the Holy Spirit, attended by twenty-five hundred or more bishops; convoked by a validly elected Pontiff who prepared schema for two years, which were orthodox, dogmatic, and definitive outlines of our Faith; the promise of Our Lord never to leave His Church; the indefectibility of the Church of Christ; all these defeat this proposition of the traditionalists."
" (2) The second erroneous proposition of the traditionalists is that the form of the Mass, frozen by Pius V, was the Mass celebrated by the early Christians in apostolic times. In answer to this erroneous proposition, overwhelming historical evidence exists for any serious student of the liturgy to know that it took almost five centuries to form the very core of the Roman Rite Mass, the Roman Canon."
" (3) The third erroneous proposition of the traditionalists is that the rite of the Mass is identified with the very sacrament of the Eucharist. Two basic facts oppose this view: (1). The very fact that there are 17 oriental rites (give or take a few variations), that are officially recognized by the official Church, and whose development also hearken back to the ancient Church. (2). The second fact that opposes this third assumption of the traditionalists is that historical evidence itself reveals the human development of the rites of the Mass."
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"(4) The fourth erroneous traditionalist proposition is that nothing in the traditional Mass needed reform or clarification. In answer to this, in fact, for the student of the liturgy, many things were confusing about the tradition a I Mass which needed clarification. (Admittedly, these items were not of the essence of the liturgy nor detrimental to the flow of the sacred action. Still, had Pius XII lived longer, he would probably have addressed these cloudy areas of the Mass, as effectively as he had reformed the rites of Holy Week in the mid '50s). Following are some examples of the areas that needed to be addressed: in an effort to show the distinctiveness of the priesthood of the celebrant of the Mass, and the "priesthood' of all believers," the traditional Mass constantly had the priest recite the same prayers and antiphons of the people and the choir, such as the Introit, 'the Gradual, the Communion verse, the Gloria, the Credo, and others. This was due, of course, to the prevalency of private and low Masses in which no choir and general public may have been present. Yet at a sung Mass, all these similar prayers were reduplicated by both priest and choir , instead of being able to be sung by both priest and people together. Another example, is that the traditional Mass, especially a sung Mass, seemed to put an emphasis on endings of prayers, e.g., the Doxology of the Secret (per omnia saecula saeculorum), the very end of the final Doxology (per ipsum...), the ending of the libera nos, etc.
This made for a very confusing emphasis on sung endings, instead of the whole prayer. The third area revolves around some "vestigial organs" of previous ancient rites whose vestiges in the traditional Mass provoked some questions on the parts of scholars. One such notable example is the priest turning to the people after the Creed to greet them and invite them to prayer, to a prayer which never follows. It is at this point in history that the prayer of the faithful, or the "bidding prayers" followed, gradually atrophied, and then only the introduction of the rite remained. "
"(5) The fifth erroneous traditionalist proposition is that the Novus Ordo Mass is invalid. This proposition is the most serious of all, for it undermines the whole indefectible nature of the Church, that She could be guilty of promoting invalid sacraments. As absurd as this proposition is, many either explicitly or implicitly hold to this viewpoint. To give some examples: Once, while giving a mission at a traditionalist parish (at the time in which I celebrated only the Novus Ordo Latin Mass with Canon #1), wherein I could not celebrate my private Novus Ordo Latin Mass on the altar of the main chapel, the chapel which had a charter and constitution that forbade the Novus Ordo Mass from being celebrated on it. Instead, I was forced to celebrate my Mass on a table in the library off from the side of the chapel, Another incident was on the occasion of a visit to a traditionalist order of sisters who claimed union with our Supreme Pontiff. But when asked what Missal I was to follow, and what Mass I was to say, and after responding that I say the same Mass as the Holy Father, was consequently refused access to celebrate Mass in their chapel. Arid I asked the dear Mother Superior, "Do you mean to tell me that if the Holy Father were to come to your convent, you would not allow him to celebrate the Novus Ordo Mass, even if he used Latin and the first Canon, as I was planning to do?" And she answered, "Yes."
These incidents clearly display an ignorance of Church history, of the history of that liturgy, of ecclesiology, and of sacramentology. For the failure to distinguish between the essential elements of the Mass common to all liturgies of all rites, and the accidental elements of other rites which are found in those liturgies is a true root cause for all of the above erroneous propositions. "
Fr. David-Ladislaus Przedwiecki, O.F.M.
In other words, the mass that had evolved over the centuries was frozen by Pope Pius V, for reasons that seemed valid in its time. Errors had crept into the rite and the council fathers wanted to allow it to breathe again. One can argue, perhaps, that they went too far.