Comparing stackoverflow and EL&U I have found that over at stackoverflow more questions remain at -1 compared to EL&U (answers are different).
The point is that for downvoting there is one reason - the question is bad. Usually a person will downvote and explain downvote and after that there is no reason for more people to downvote it.
The asker now has the option to improve the question.
Also, new users might not be able to delete their questions (?) and also get even more frustrated against improving.
It is also easier to look at questions that had a problem (bad questions that had activity).
Finally, if question is really bad, not improving, etc it should be further downvoted.
Let me just mention that this has nothing to do with closing, questions can be closed even without downvoting at all.
Although thinking about improving EL&U is the proper motivation for downvoting a question I think that downvoting more than once, actually, does not have that effect.
Here's a post that triggered the thinking for those interested. Also, this image compares the ratios of question scores: