I recently posted a question which was closed and then received several down-votes after being closed and having chosen an accepted answer. I tried to delete the question but SE doesn't allow this. So my question is, why does SE want to keep what's clearly a low quality question? And as a user, how is it fair that I'm receiving down-votes for something I have no control over?
To answer the question in the title:
Why does Home Improvement prevent my question from being deleted if its been closed and downvoted?
Because (apparently, by the three upvotes and the accept vote) it produced a valuable answer, which somebody took the effort/time to write. It would not be fair to them if you could delete your question just because you received a few downvotes. (Two is 'a few', not an 'assault'.) Looking at it from another way, you even gained reputation from that question (one upvote compensates for five downvotes).
What do the down-voters expect me to do?
I don't know (I can't read minds), but since they all happened after you posted a comment referring to them as 'elitist mobs', one simple thing would be not to use terms like that. Decent/interesting questions that are not suitable for the site are closed, but not necessarily downvoted.