Probably because XML would need to be extended quite a bit to accommodate all of the multimedia stuff, attributes without values or quotes, special names for certain characters, optional or disallowed closing tags and whatnot that's in HTML5.

I think pushing in both the layout design conveniences and the strictness of XML data transfer in the same standard would be quite bulky at best. In practice we'd likely see a lot of nasty security issues in implementations and so on.