<p>As the other question did not address the <em>why</em> really well, let me summarize what I found on the matter.</p>

<p>This has already been asked on <a href="http://meta.stackoverflow.com/">Meta Stack Overflow</a>:</p>

<p><ul>
<li><a href="http://meta.stackexchange.com/questions/21020/how-to-delete-closed-question">How to delete closed question</a></p>

<blockquote>
  <p>I would like delete a question of mine since it's been closed. Is there a way to delete a closed question? I am owner, so why am I not able to delete it the normal way? Do I have to ask some moderator?</li>
  </ul>
  People are usually referred to <a href="http://meta.stackexchange.com/questions/5221/how-does-deleting-work-what-can-cause-a-post-to-be-deleted-and-what-does-that">How does deleting work? What can cause a post to be deleted, and what does that actually mean?</a>. Specifically, the section:</p>
</blockquote>

<h2>When can't I delete my own post?</h2>

<blockquote>
  <p>You can't delete <em>answers</em> that have been <a href="http://meta.stackexchange.com/questions/5234/accepting-answers-what-is-it-all-about">accepted</a>.</p>
  
  <p>You can't delete any <em>question</em> that:</p>
  
  <ul>
  <li>has an <a href="http://meta.stackexchange.com/questions/74466/does-systematic-self-deleting-need-to-be-prevented/74471#74471">upvoted answer</a>, or</li>
  <li>has multiple answers (even if there are no upvotes), or</li>
  <li>has been <a href="http://meta.stackexchange.com/questions/56149/why-cant-you-delete-your-own-recently-closed-posts">closed less than 48 hours ago</a> (to allow for possible reopening)</li>
  </ul>
  
  <p>You can't delete <em>any</em> of your questions or answers if you haven't <em>registered</em> your account (i.e. associated an OpenID with it).</p>
</blockquote>

<p>So the reason is simply: <strong>to allow for possible reopening</strong>.</p>