What could be the cause if both have upvotes?
Upvotes on the question are irrelevant in this case. Read again:
This questions does not have an upvoted or accepted answer.
Questions can only be closed as a duplicate if the question it's a duplicate of has already been answered. This answer must have an upvote and/or be accepted.
As Donald.McLean indicated, closing the unanswered question as a duplicate of the answered question worked perfectly fine. The age of the question is irrelevant.
In a Q&A way, this makes sense. The reason duplicates are closed-and-linked is to make it easier to find your answer. So all the links better point to the question with the answer, instead of having questions with answers linking to questions without them. That's kind of useless after all.
Now, the questions with an answer can receive additional answers which either clarify the matter or give an alternative. Anyway, this couldn't be done if the question is closed.