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:
- 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?. Specifically, the section:
When can't I delete my own post?
You can't delete answers that have been accepted.
You can't delete any question that:
- has an upvoted answer, or
- has multiple answers (even if there are no upvotes), or
- has been closed less than 48 hours ago (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.