The fact that Internet Explorer doesn’t really support XHTML as XML in any way, and the problems XML can cause when not all tools in the authoring chain are XML tools, means that there has been little incentive for using XML on the web. This is compounded by search engines not indexing XHTML as XML documents; very few XHTML authoring tools for XML; very few CMS or blogging tools supporting XML correctly all the way from input through database to generation; and very few ad suppliers supporting XML.
There is a little incentive if you want to allow MathML, SVG, and other XML applications to be interspersed inline in XHTML documents, but this use of XHTML as XML has found a very limited audience.
XHTML2 is XML
And therein lies the biggest problem. On top of all the concerns that web developers have about using XML for serving documents, XHTML2 adds another layer of complexity. It isn’t HTML 4.01 reformulated as XML; it’s a different but similar language, with added, removed, or modified semantics for many elements, and added or changed element vocabulary for many semantics. In many cases, the changes are steps in the right direction, but at the same time, XHTML2 was not built with web developers in mind. As an example, it doesn’t at all address the deficiencies of HTML 4.01 and XHTML 1.0 in the areas of interactivity, local storage, or script interactions.