Because HTML is a markup language. XML is also a markup language but people somehow insist on using it for things where a markup language is not appropriate (which is almost everything).

XML is rarely used as a markup language, in fact it is almost a failure as a markup language, witness the non-use of XHTML. It is a markup language that turned out to have more utility as a general purpose data format, and it certainly didn't get that way by people using it for applications it was not intended to support.

Because in HTML it's clear what's an attribute and what's content

In the case above, not so much (though I'd say it's the artifactId)

But since XMLers like waddling in verbosity and bureaucracy it seems they shot their own foot with it