I think the idea is that if the older question has no upvoted or accepted answer, then it may well be the case that the newer version has done a better job of asking the question, in which case it would be preferable to close the older one as a duplicate of the newer one. This view is expressed by Gilles. I think this motivation is reasonable.
Many are opposed to the policy; what I find to be good arguments against it are given by fbueckert:
This new rule seems to encourage dupes of unanswered questions, which, if no one can answer the question, means we just gather un-closable questions. I suspect we'll shortly get answers that do nothing but go, "I don't know", and get upvotes, just so we can close them.
What happens when one of the questions gets an answer? The effort to find the duplicate earlier goes wasted and someone else has to find the dupes yet again after one is answered, and know which one is answered just to be able to close.
The feature request on meta.SO asking that this policy be overturned was declined, so I doubt there's much chance of it changing in the near future. In the meantime, I think a reasonable solution is that, if you find duplicate questions none of which can be closed as a duplicate of any other, to post links to the others in the comments, so that they can be easily noticed when necessary. If you think that there's a strong reason to close something as a duplicate and you're unable to do so because there are no answers, that's when I think it'd be okay to flag a moderator; if there are answers but none with upvotes, just give one of them an upvote, it's not a big deal.