Because critical features that users need aren't there. Eg: https://news.ycombinator.com/item?id=32248514
The wider audience wants to be able to hand out their contact details at a bar and know they'll be able to block the person if they turn out to be a creep.
Or they want to be able to block their annoying ex, that crazy QAnon fan cousin etc etc.
This is a 100% must have feature, and yet it's treated like an optional feature.
There's so many features like this - it's like people using XMPP have never looked at how most people use messaging.
However I doubt this is the reason why the most popular messengers do not comply with the XMPP standard today.