Not the official source, but it seems to me like this criterion was added as a result of Jeff going through lots of queries and sorting out potential criteria for "noise" unanswered questions, and finding that questions with scores of 1 that were posted by deleted accounts added to the same level of noise that questions with zero scores did.
The intent of that overall deletion criterion is to remove old, unanswered questions from search results, because such questions are just basically noise to other users searching for answers; see https://xkcd.com/979/.
There are probably many reasons why Jeff arrived at the query result he did. For instance, if a user deletes their account, it's probably the case that they don't care very much for their question getting an answer, than if they are still active on the site. Answers to questions with lower scores don't have as much of a reach as those to higher-scoring questions, and at such low scores where the answer would more likely be more beneficial to the author than the overall Internet audience, it makes sense to clean up the "noise" questions where there isn't an author around to benefit.
Another reason could be users who post a question that has no answers and zero score after a while, then they delete their account, but later rejoin the site; upvoting their old post through their new account will not prevent it from being deleted.