The GIRM gives some history on the 15th century council’s important, but misguided determination:
11. The Council of Trent had already recognized the great catechetical usefulness contained in the celebration of Mass but was unable to bring out all its consequences in regard to actual practice. In fact, many at that time requested that permission be given to use the vernacular in celebrating the Eucharistic Sacrifice. To such a request, the Council, by reason of the circumstances of that age, judged it a matter of duty to answer by insisting once more on the teaching of the Church as had been handed on, according to which the Eucharistic Sacrifice is in the first place the action of Christ himself, whose inherent efficacy is therefore unaffected by the manner in which the faithful participate in it. The Council for this reason stated in these firm and likewise measured words: “Although the Mass contains much instruction for the faithful people, it did not seem to the Fathers expedient, however, that it be celebrated indiscriminately in the vernacular.”[12] And the Council declared worthy of censure anyone maintaining that “the rite of the Roman Church, in which part of the Canon and the words of consecration are pronounced in a low voice, is to be condemned, or that the Mass must be celebrated only in the vernacular.”[13] Nevertheless, at the same time as it prohibited the use of the vernacular in the Mass, it ordered, on the other hand, pastors of souls to put appropriate catechesis in its place: “Lest Christ’s flock go hungry . . . the Holy Synod commands pastors and each and all of those others having the care of souls that frequently during the celebration of Mass, either personally or through others, they should explain what is read at Mass; and expound, among other things, something of the mystery of this most holy Sacrifice, especially on Sundays and feast days.”[14]
[12] Ecumenical Council of Trent, Session XXII, Doctrina de ss. Missae sacrificio, September 17, 1562, chapter 8: Denz-Schön, no. 1749.
[13] Ecumenical Council of Trent, Session XXII, Doctrina de ss. Missae sacrificio, September 17, 1562, chapter 9: Denz-Schön, no. 1759.
[14] Ecumenical Council of Trent, Session XXII, Doctrina de ss. Missae sacrificio, September 17, 1562, chapter 8: Denz-Schön, no. 1749.
In hindsight, it is easy to characterize this as a political judgment in search of a theological justification. Trent accomplished much, but the bishops convened for it also missed opportunities. Even if they did provide for the homily on Sundays and feast days.
Catholic Sensibility 
It wasn’t a “political” justification, it was a theological one against specific heretical propositions of the Reformers. Do you have any historical evidence that any contemporary Catholic head of state objected to Trent’s Doctrine Concerning the Sacrifice of the Mass Chapter VIII and solemn canon 9?
What, by the way, in your opinion was misguided about the Tridentine decree?
It strikes me as politics, pure and simple. Some bishops couldn’t abide being told that medieval practice was wrong, and they took a stand to stand apart from Protestant reformers. That’s not theology. It was an unpastoral judgment in search of a theology. Like the separation of the initiation sacraments in antiquity. Like the minimizing of Communion from the Cup.
It has nothing to do with secular politics. Politics is part of the human condition. Two or more people always have the potential to play politics. That it happens in the Church, even among bishops, isn’t unexpected or even necessarily bad. My view is that we should recognize it, if for no other reason than to ensure it doesn’t run the show.
The Tridentine decree on Latin only was misguided. Relying on preaching skills that were very weak in most clerics was not a wise or prudent call.
No, the Fathers of Trent took a stand because the Reformers’ theology was heretical. Trent is nearly an ideal council of clarity where the normative theology was written in the body of the chapters while the heretical notions were precisely condemned in the solemn canons. That way evil could be clearly identified while not limiting doctrinal development elsewhere.
Glad to see your clarification though that the decree was free from secular politics. Truth be told, that is how I took your post. Now I see that my take was a misunderstanding of what you meant by the reference to political.