Apparently, the W3C just isn’t getting enough uptake on XHTML, because it’s simply too hard to shove people in the right direction:
The attempt to get the world to switch to XML, including quotes around attribute values and slashes in empty tags and namespaces all at once didn't work. The large HTML-generating public did not move, largely because the browsers didn't complain.
As a result, the W3C will be making incremental improvements to good, old HTML for a while yet. Thankfully, they’re also planning on updating the HTML validator.