Why doesn't XML allow nesting of comments

Question

Is there any particular reason (other than it would make the parsing harder) that XML doesn't allow nesting comments, i.e. something like

<!--
  <!-- this is the foo element -->
  <foo>bar</foo>
-->

?

Answer 1

It would have been entirely possible to allow nested comments, without changing the syntax. The designers of XML chose not to because they were greatly concerned with SGML compatibility: it would have allowed XML documents that were not valid SGML documents.