Why XML Schema Description Language?

Technology and Politics

Document Type Definitions (DTDs) XML grew from SGML, which had really be a set of tools for managing documents. As XML use spread among a wider range of programmers, it became clear that the document-oriented tools provided by DTDs weren't up to the challenge of describing more complex kinds of data structures.
XML syntax to describe XML documents DTD syntax, unlike XML document syntax, is not easily extensible. DTD information has also proven more difficult to manage (or perhaps groups haven't been interested) with tools like the Document Object Model as well. DTDs are structured differently from XML documents, as well.
W3C Initiative The World Wide Web Consortium, as 'keeper' of XML, felt strongly that building a schema language for XML was a task that it should take on.