As the other question did not address the *why* really well, let me summarize what I found on the matter.

This has already been asked on [Meta Stack Overflow](http://meta.stackoverflow.com/):

- [How to delete closed question](http://meta.stackoverflow.com/questions/21020/how-to-delete-closed-question)
  > 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?

People are usually referred to [How does deleting work? What can cause a post to be deleted, and what does that actually mean?](http://meta.stackoverflow.com/questions/5221/how-does-deleting-work-what-can-cause-a-post-to-be-deleted-and-what-does-that). Specifically, the section:

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

So the reason is simply: **to allow for possible reopening**.