I have posted a question Which down-voted and closed. Now I want to delete the question. However I am getting error. Why is such a restriction?
Error which I am getting
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Your question has multiple answers; this is an indication that perhaps the question is suitable for reopening.
If your question was not closed, you'd not be able to delete the question at all. Because it is closed but has answers on it, the delay is there to give someone the chance to improve the question, perhaps to the point that it can be reopened.
If this doesn't happen in the next two days, you can self-delete it. It'll be auto-deleted after some time anyway, if none of the answers gain upvotes or the question has not been edited for at least 9 days.
See How does deleting work? What can cause a post to be deleted, and what does that actually mean? What are the criteria for deletion? for details on when questions are auto-deleted, or under what criteria open questions cannot be deleted.