"Anything but Christianity" is axiomatic for modern mainstream culture. In other words, whatever the attitude towards religion and spirituality (from broad sympathy to total atheistic rejection); there is a special animus directed-against Christianity.
And few are even explicitly aware of this in themselves.
I speak from experience. For several decades before I became a Christian, I was a 'spiritual seeker'; and had regarded myself as (in some general way) broadly in favour of some aspects of Christianity; and I had read (and thought about) comparative religion, mythologies of the world, modern spirituality (New Age), and even modern Christian theology.
Yet, in truth, my was attitude that I was looking for An Answer everywhere except in Christianity.
When reading Christian material, I was always pleased to see it reinterpreted in the light of other religions or traditions. "If only Christianity were more Buddhist, or New Age spiritual - if only Christianity could admit it is just one of many paths to truth - then I might find a home there" was the sort of idea.
But at the root of it, as I now see that what I was objecting to was resurrection. I did not want to be resurrected after death; I did not want to stay myself, in an eternal body: I did not want to repent my sins and dwell in a Heaven among other such resurrected Beings.
Of course I didn't believe resurrection was even possible! I thought the idea was (very obviously!) childish wishful thinking to be found only among the weak-minded; or that nobody really believed it but pretended in order to reassure themselves or manipulate others.
But believing that resurrection was impossible nonsense was not the root of it: the root of it was that I rejected it. And, because I rejected specifically what Jesus Christ achieved and offered us; naturally I rejected Christianity.
I think that my own ex-views are not uncommon; I believe that many of those who reject Christianity do so in a specific way (i.e. "Anything but Christianity"); and that many of these people reject Christianity because they personally do not desire resurrection.
Many desire, instead, some form of self-annihilation: whether the mainstream atheist-materialist view that the subjective-self ceases when the body dies; or the Western-Eastern religious view that our self-awareness dissolves (back) into the universe and a state of blissful Nirvana; or that we persist after death in an immaterial, disembodied form - spirits, ghosts, or whatever. To all of these; the offer of resurrection is unwanted, is rejected.
A few people desire to reincarnate into mortal bodies on this earth; to come back over and again, and relive this kind of temporary life among in this entropic world; and these do not desire the final answer of resurrection with an eternal-body.
In other words; the trend for unpopularity, and the increasing scarcity, of Christianity is rooted in what modern people do not want. As things stand, modern people do not want what Christianity has to offer - quite aside from whether they believe that the offer of Christianity is a true and valid one.
The question is why modern people nowadays do not want and reject that which was, a couple of thousand years ago and for many centuries, regarded as the greatest possible Good News.