Because of Leftism. In particular because men come to recognize the moral authority of the Leftist establishment.
People don’t lose their faith; they switch their faith. Christianity is disappearing because it faces a confident, aggressive, proselytizing rival. Changes in material conditions, personal shortcomings of Christians or their clergy, failures to adequately appeal to this or that faculty (the intellect, the imagination, the intuition,…)–these are at most secondary and negative causes. That is, they did not cause the downfall of Christianity, but at most may partially explain why Christians were not strong enough to resist the Leftist counter-faith that actually accomplished the destruction.
Imagine someone trying to explain the decline of paganism in fourth-century Rome invoking only intrinsic weaknesses of the pagan cults and not mentioning Christianity.
One hears silly things about the late heretic Hans Kueng, such as that he was a promising theologian who was led into conflict with the Church because he would not submit his mind to orthodoxy, as if he were some sort of free-thinker. In fact, Kueng was slavishly obedient to an authoritative magisterium, just not that of the Catholic Church. Show me one of his famous dissents which was a dissent from the Leftist establishment, rather than an affirmation of it. Are there any non-Leftist heretics these days? A Catholic theologian who idiosyncratically free-thinks his way into Monophysitism or Nestorianism–that I could respect.
Consider Bruce Charlton’s 2020 ideological-religious Litmus Tests (briefly: coronavirus, antiracism, and climate change). He writes
If you support any or all of these; you are objectively on-the-side of mainstream, global, totalitarian Leftist Establishment: which is the side of Satan and against God. And obviously, therefore, you are anti-Christian – despite whatever you may believe or assert.Bruce Charlton’s Notions: http://charltonteaching.blogspot.com/2020/10/in-2020-you-are-objectively-leftist.html
This is an extraordinary claim, because he chose (I believe deliberately, to make a point) three bits of establishment orthodoxy that don’t directly contradict Christian orthodoxy. A Christian logically could accept any one of them without contradicting his doctrinal confession. Indeed, idiosyncratic Christians of past times might have accepted some of them. Las Casas thought that whites were being mean to non-whites. A Christian chemist from a century ago might have guessed that the effect of increasing concentrations of greenhouse gasses in the atmosphere would be to warm the surface without thereby imagining that he was defying his religion.
However, at least most of the time, the Litmus Tests work. Those who accept them usually turn out to be more deeply compromised, in that they are seldom willing to contradict the Left where it unambiguously does contradict Christianity (e.g. sodomy issues). The reason is that most people accept the Litmus Test beliefs, not because of their personal experience with interracial relations or their expertise on radiation transport or epidemiology, but because they have been told to accept these beliefs by an establishment whose reliability and authority they recognize. That same authority tells them to accept the whole Leftist moral hierarchy, and in particular that any person or tradition that deviates from it is wicked.
Establishment-sanctioned beliefs are peculiarly ad hominem. The mob is energized less by “X is true” than by “those who doubt X are evil/stupid/low-class/low-status”. The Left claims a monopoly on reason, virtue, and class, and these claims are almost universally accepted. The point deserves emphasis. Leftism attacks Christianity not only by asserting moral claims that are incompatible with Christian orthodoxy, but primarily by attacking the character of past and present generations of Christians. Christians are too quick to grant their personal sinfulness–and, less creditably and more impiously, that of their ancestors–in order to concentrate on what they think are the fundamental disagreements on metaphysics or ethics. Thus, the battle for moral status is surrendered without a fight, sometimes merely “for the sake of argument” (i.e. not to distract from those “real issues”) but more often from a misplaced sense of genuine guilt. However, men are social creatures, not rational creatures. The battle for moral status is the most consequential one, for most people will accept any high moral status proposition, regardless of the logical strength of the arguments for and against it.
Many of our priests think they can win back the good graces of their people by embracing Leftism-compliant beliefs that are at least compatible with Christian orthodoxy. Global warming is a popular one. Why shouldn’t a Christian support action against global warming? Well, he certainly can; perhaps it will do the environment some good, but he should not fool himself that it will do the Church any good. As the Left ever more stridently demands that everyone embrace their hierarchy of practices and peoples (e.g. sodomy good, white people bad), the only Christians left will be those who consciously say “no” to the Left, at least in their own minds. The authority of the Left must be defied. It’s not enough to show that the Church is right where she agrees with the Left; we must establish that the Left is wrong on some point where she disagrees with the Church. Of course, an orthodox Christian will say that the Left is wrong on all matters where she disagrees with the Church, but even to establish solidly on one issue that the Left is wrong is bound to have a tremendous effect on a soul. That soul realizes, despite the lifetime of indoctrination we’ve all had, that the benevolence of progress and the correctness of establishment opinion cannot simply be assumed. The authority of Leftism has been put into question. The issue doesn’t have to be sex if you’re squeamish about sex, but it has to be something just as uncomfortable as sex, because it requires agreeing with the Church when she disagrees with “all reasonable people” and “all decent people”.
A final observation. It’s a common thing for reactionaries of our stripe, as they read and think themselves deeper into the Right, to show increasing scorn toward what passes for conservatism in modern America–Republicans, National Review readers, Trump voters, and the like. “They’re just embracing yesterday’s radicalism; they don’t really disagree with the Left at all on fundamentals.” That’s true, but they really do think that they disagree with the Left, and that is something quite important. However intellectually compromised they are, they have all thought, spoken, or voted in some way that they were clearly told all reasonable/decent people regard as absolutely unacceptable, and they did it anyway.