lowercase XML languages

Why are all the XML languages lowercase?

Up until I attempted to create XHTML I had a nice way to show HTMLers what
was HTML and what was XSLT in an XSL stylesheet. The XSLT is all lowercase
and the HTML is all upper case. They can quickly glance at the XSLT and see
what they need to affect. By forcing me to use lowercase for everythinig you
have eliminated this simple/basic separation of concerns.

Is this decision arbitrary or are there reasons?

best,
-Rob