XML spec and XSD

Hi all,
   Seeing the XML 1.0, 5th edition spec (ref,
http://www.w3.org/TR/REC-xml/) I cannot find any reference to the
phrase "XML Schema" or XSD (which are the XML Schema languages,
defined by the W3C specs viz specified at,
http://www.w3.org/XML/Schema).

The XML 1.0 spec, certainly refers to the word DTD and provides the
complete definition of DTD semantics, and how it integrates with XML
as a validation technology. This all is great.

But I wonder, why the XML spec (1.0 or 1.1) doesn't mention XSD (i.e,
W3C XML Schema language) as also a second XML validation language
(i.e, other than DTD)? I am curious to know, was this decision not to
mention XSD as a validation technology in XML specs, was a consicous
one? If yes, what advantages we have achieved by this decision?

This curiousity spurs to my mind, keeping in mind that XSD is a very
important XML validation technology today, which is a W3C standard and
implemented widely in software applications.


-- 
Regards,
Mukul Gandhi