I struggle to understand why XMPP isn’t the de facto messaging standard around the world,

I suspect that normies subconsciously want all popular messaging services to be insecure.

Humans are often self-confident in a bad way about imagined chaos.

So if a service in insecure, then some day (as that dreaming normie thinks) they might "come to darkweb" and find "a hacker" to help them against something bad. After all, that's how movies show life.

And if a service is secure, then, that normie thinks, it's either a scam or at least something that movies have not prepared them for. Same with encrypting mail.