The reason is simply that it has been around for generations, but never gets any further; also I see no evidence that it has any significant tendency to strengthen and improve its adherents - whom seem to just as susceptible to the pathologies of modernity as is anyone else (and indeed among the leadership of Perennial Philosophy, the standards of behaviour - when judged by traditional values - are, on average, perhaps lower that the population average).
I have returned to this matter through my current project of reading Jeremy Naydler, and seeing that he teaches at the Temenos Academy, which is perhaps the most prestigious of Perennial Philosophy organizations - since it is actively supported by many establishment figures including HRH The Prince of Wales (presumably our future King).
What is the Perennial Philosophy? Here is a video self-explaining:
My own pragmatic definition is that the movement was (historically) set-up by, and continues to attract, disaffected ex-Christians and intellectuals who have positive views about most religions, spirituality, and selected aspects of some actual churches; but not about what they term 'fundamentalist' Christian churches. The PP is thus, pretty much, the intellectual wing of the New Age movement.
The basic idea of the PP is that all world religions (properly understood) share a core body of beliefs, and this core is true and the most important aspect of them, and these core truths can-be and ought-to-be the basis of the highest and best kind of religion/ spirituality.
I personally agree that all real religions share core truths - but I do not agree that what is shared is necessarily the most important thing - and especially not for these times. I certainly do not agree that the shared core it is the best kind of religion/ spirituality - and in practical terms I think it has been empirically proven by experience that these shared core things are not a sufficient basis for a strong and good lived religion/ spirituality.
In sum, the Perennial Philosophy seems too weak, too feeble, too diffuse and easily dispersible to be the core of a human life. And indeed, adherents do not behave as if the PP is the core of their lives; rather, they fit it into other priorities.
So, Perennialists tend to be positive (in a semi-detached way) about Eastern religions generally (especially Buddhism, but also some aspects of Hinduism such as the Bhagavad Gita - although not the caste system! - and especially keen on Taoism; also Sufism, sometimes Kabbalism, and the mystical traditions of Christianity; such as Eastern Orthodoxy, the meditative tradition of monasticism in the Roman Catholic Church, Gnosticism, Neo-Platonism.. and many types of ancient paganism, animism, totemism and... have I missed anything?
Among Christian churches PP-ers do not seem to like Protestants generally and Lutherans, Calvinists, Methodists, Baptists or modern conservative evangelicals or Pentecostalists in particular... in sum they show no interest in the currently growing parts of mainstream Christianity. And they are either bored-by, or hostile-to, newer and thriving churches such as Mormons, Jehovah's Witnesses and Seventh Day Adventists.
As so often, there is a cleavage line around the sexual revolution. Perennial Philosophy advocates are almost all advocates of, and adherents to, one or more aspect of the sexual revolution, and the expansion of sexual relationships beyond religious concepts of marriage. This is very obvious among both leaders and followers - and the PP environment is often sexualized; with multiple marriages, cohabitations, affairs and the rest of it - in line with mainstream modern secular life.
Furthermore, in their political views they are Leftist - often the ultra-idealistic Leftism of Anarchism, Deep-Green ecology, Voluntary Simplicity etc. They may or may not support mainstream Left parties (often they do - but some ultra-Leftists do not); but they never support 'Right wing' parties - even by the insipid mainstream standards of mainstream politics whereby what would have been hardline Leftism two generations ago is now supposed to be far Right!
In general, the PP adherents never, in their analyses, regard the problems of the modern world as a consequence of Leftism - despite the close temporal correlation; but almost always as a consequence of something described as Right Wing - whether capitalism, libertarianism, industrialization, patriarchy, racism, colonialism etc.
In terms of their lifestyle, Perennial Philosophy advocates have adopted most of the post-sixties markers of New Left modernity - 'alternative'/ bohemian styles of grooming and clothing, ponytails and ear-rings for men, body piercings and tattoos with increasing popularity, green consumerism, a focus on Africa and a pro-ethnic stance, alternative- and psycho-therapies, Fair Trade products and vegetarianism, an interest in 'renewable' resources, 'organic' horticulture and farming, Anthropogenic Global Warming and so on, and so forth.
My point is that the Perennial Philosophy has been around for an awfully long time now - at least three generations of significant visibility (since, say, the Beat Generation of the 1950s), yet there is no evidence that it makes any significant difference to the lives of its adherents - they do not stand-out against the currents of our time in any way which would lead to significant problems for them.
On all the litmus test issues of our time - the kind of things that could get you sacked or made a public pariah - i.e. the issues of Political Correctness - the Perennial Philosophers are all comfortably on the side of the secular mainstream.
This must be either a terribly lucky coincidence! - or else demonstrates that the PP has no real backbone, no capacity to en-courage and strengthen its leaders or followers in face of adversity; but is merely a weathervane for mainstream secular Leftism.
On the one hand I still read and enjoy, and am not much worried about the Perennial Philosophy; on the other hand I don't expect anything substantive from it. Despite its hostility, it is no real threat to real Christianity. No doubt, as a half-way house it is sometimes a distraction from potentially better things; yet it may also be a stepping stone to better things (as it was for me).
So while I am not hostile, and I often find much to interest me in the work of Perennial Philosophy advocates - for instance, there are often good diagnoses of the deep problems of modernity even when the prescription and treatment-plan is usually worthless or just more of the same-old - I cannot be optimistic or enthusiastic about the future of the movement.
I see not the slightest evidence that the PP can be a real religion; and on the contrary I do see a recurrent pattern of hopes leading nowhere.
In sum; the Perennial Philosophy is at best an intellectual activity - part of the History of Ideas, more typically a feel-good Lefty lifestyle; but it is not a thing capable of deeply-motivating, nor of powerfully-inspiring, its adherents.