Because they Do.
Naturally, people care more about religion than any other 'big idea' - more than about politics, nations... and certainly more than about their own comfort, convenience, peace and prosperity.
But it is probable that the reader finds this an incredible, perhaps ridiculous, idea. Probable that he looks within himself and observes: "Well, I don't care much or anything about religion!"
That may well be true. Because this blog is read mostly in the West, in developed nations, among the intellectual or leadership class - and this class of people (including you, me...) are mentally sick.
They (we) are insane - as can be seen from their behaviour; which is not just passively indifferent to the primary realities of life (marriage, family - as well as religion); and passively doing nothing to sustain the living conditions they supposedly value (for example science, technology, education, law have all been allowed to become destroyed, utterly, by politics) -- but they are also strategically destroying themselves and their living conditions by multiple active means...
(I mean: Politically correct leftist policies of many types from socialism, multi-culti, diversity, antiracism, and the sexual revolution; through mass im/ migration, through the global warming fraud, to deliberate incitement of civil and international conflict by systematic propaganda of resentment, entitlement, fear...)
In fact - religion is the One And Only Big Idea that people naturally, spontaneously care about.
Delete religion (or alternatively corrupt, subvert and invert it) and the result is simple demotivation and despair - alleviated by self-distraction, aimed-at suicide.
Our culture has deleted religion. Modern political 'substitutes' simply don't work - because nobody really cares about politics.
They just don't - politics is almost-entirely a mixture of covert selfishness, idle gossip and projection; which are insufficiently strong motivators - and insofar as politics is a real motivator it is feeble, unstable, subjective - useless for the heavy-stuff of living.
Why do people care so much about religion?
Why wouldn't they? Because religion is real (albeit distorted fragments of reality); and religion is about truth, life, consciousness - the fundamental assumption - and these are necessary.
And nothing else except religion even tries to provide them.
Of course there has been frequent, massive and hostile disagreements about religion in the past... but never the idea that religion didn't matter.
Never that religion was purely subjective.
Never the idea that religion could simply be deleted from Man and Society, and this deletion would actually improve Man and Society!...
That's just crazy. As religious people can see all around.
It amounts to the cardinal sign of insanity - lack of insight... The non religious are so crazy that they cannot perceive the fact - it's the one fact they cannot perceive; despite the chaos in their minds and the chaos all around.
So crazy that each individual supposes he is the only sane thing in a mad universe...
And the escape begins with recognising that fact.