Many valid reasons have been given in this thread. I suggest you consult the World Christian Encyclopedia
en.wikipedia.org/wiki/World_Christian_Encyclopedia , and World Christian Database
worldchristiandatabase.org/wcd/ to check the growth of non-Catholic denominations during the same time frame. There has been an explosion in non-Catholic and non-Christian sects, all of which target the Catholic Church for members.
Consider that the Catholic Church has the most stringent moral teachings, during a period in history when world moral behavior is rapidly declining. I would imagine that a major decline occurred in the post-World War II period, as troubling social demographics seem to follow involvement in war. Two other wars quickly ensued after WWII, both greatly affecting the American Church.
To blame it on the NO mass is a cop out. I have never attended a Tridentine mass, and firmly believe that, if one is even moderately well catechized, is seeking a depth of faith, and is aware of what is occurring at the mass, its exact form is not important.
On the grand scale of things, this is the work of the evil one. Consider how so many nations around the world, in a relatively short time, have embraced contraception and are now embracing abortion, and are considering euthanasia and institutionalized homosexuality. None of these practices was ever a major factor in world culture until very recently. With the extremely varied cultures and societies involved, how is one to explain this phenomenon? There is a larger plan at work, and it seeks to draw members away from the Church, and to involve those members (and even non-members) in what is objectively mortal sin. Beside God, who else has always had a desire for souls?
Essentially, the evil one has perpetrated a “perfect storm” against God, with the Church standing at the forefront of the fight. Among all of the other factors, notice that the world is now promoting homosexuality and its teaching to children as a “normal, alternative lifestyle”, but condemns the Catholic Church when homosexual Priests exhibit that exact behavior!
I think that a universal Latin mass would be even more severely attacked by the world, since so few people comprehend Latin. The use of Latin would stand as evidence to the world that the Church was “out of touch”, “irrelevant”, “Patriarchal”, “sexist” or worse. Notice that those criticisms are occurring even with the Novus Ordo!
Christ’s peace be with you.