No dynamic server-side templating language, easily transformable to other XML formats (docbook, atom (see front page of registry)). If the user/client has libxml, the processor command is already installed. The specifications are now provided in an application specific format that is language-agnostic instead of manually maintained ad hoc HTML. The extension registry index page is now automatically generated into static HTML instead of manual HTML+JavaScript for sorting the lists.
Finally, the restricted power of XSLT is an advantage for this application vs PHP, ASP, or JSP. If browsers actually implemented web technologies like XSLT, sending XML specs and generating HTML on the client side with cached XSLT would save bandwidth.
Note: You can generate HTML using JavaScript but it is not needed. You can manipulate the DOM to change what the user sees.
...there are a million oft-argued reasons why not, but that's one line of thinking that will lead you to do it.
These discussion always remind me of my favorite old Slashdot quote: "XML is like violence. If it's not solving your problem, you're not using enough of it."