HTML and XML took off for web. That's what the vast majority of devs write and that's how all devtools display the DOM.
> If you do have to roll up your sleeves and work with the serialized syntax, you're far better off it its is S-expresions.
Convince the masses, because they don't seem to agree.
Thus, no structural, recursive syntax for document editing has "taken off".
Word processing is still done in Microsoft Word by most of the planet. HTML e-mails are written in some WYSIWYG client program. Web forums use variations on markdown (with raw HTML only as an escape hatch).
People do work with *ML by hand, but largely reluctantly.
If you do have to roll up your sleeves and work with the serialized syntax, you're far better off it its is S-expresions.