There is more than one answer - but an important factor is the psychological consequences of modernity: which is functional specialization.
From my book Thought Prison (2011)
The millennium-deep roots of political correctness
The West took a turn toward legalism, logic and bureaucracy around AD 1000 (‘the Great Schism’) when the Roman Catholic Church (gradually) broke away from the Eastern Orthodox Catholic Church the Byzantine Roman Empire with its capital in Constantinople.
Like most long term policy mistakes the East-West schism was initially richly rewarded (otherwise the mistake would not have been made) – since it led to tremendous 'progress' in first philosophy and scholarship (especially Thomas Aquinas) with the development of universities, then later in science and technology, and later still in the economy.
But schism led on to more schisms, with no end in sight.
But the benefits and the mistake were alike in being built-on continuous specialization of function; progressive specialization of all functions, with limit.
The Great Schism built-into the thought systems of the West a fatal error, of which PC is a remote and indirect consequence.
Once modernity (progressive specialization) has been built into a system, it cannot stop itself - and it cannot stop itself because continual specialization creates (or indeed itself is) continually increasing autonomy, so there is no way of one part of the system stopping another.
You just get more and more specialization of function until the whole social system falls apart into useless fragments; and all the King's Horses and all the King's men cannot put Humpty together again.
By secularizing knowledge, by creating The University – by making philosophy autonomous of the Church (instead of having learning institutionally focused in monasteries) the West eventually made political correctness - which is now everywhere and inescapable.
And PC is the West's Nemesis, because the West cannot decisively overturn PC without overturning that which made it The West.
The West cannot overcome PC without ceasing to be The West.
Yet, if this overturning of PC does not happen, then the West will itself be subverted by PC.
In other words, The West is built upon error: its strength is also its weakness; its power is also that which is self-destroying; even as The West built its great structures it was simultaneously gnawing at their foundations.
Here is the double-bind:
To be anti-PC is to be anti-The West (always in tendency, albeit not by intention)
Yet at the same time, to be pro-PC is to be anti-The West (always in tendency, and also by intention)
And/ So/ But The West never was sustainable.
Looking back, The West was a blip on the graph of history - albeit a thousand year 'blip'! –with a hundreds year long, gradual up-tick and the rapid decades long collapse looming ever-closer.
Just so soon as The West began to implement its assumptions towards completion (which is political correctness), just so soon The West began actively (as well as passively) to destroy itself.