I got two down votes this for my question that was closed 4 months back, I have also marked it for deletion. then why downvoting?
This answer on meta.SO explains the reasoning. The basic idea is that the question may also get up votes, votes to reopen, edits, and so on that might salvage it. It takes sufficient delete votes to establish the question as unsavable and end the process.
If you'd like, I'll delete the question for you. It just didn't get enough delete votes.